How can I prove this difference satisfies this condition? [closed]

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Consider the following polynomial difference $A(x)-B(x)$:
$$A(x)=x^4-6x^3+8x^2+6x-9$$
$$B(x)=x^4-6x^2-8x-3$$



How can I prove/know that this difference satisfies this condition:
$$A(x)-B(x)<0 iff x:in :]-1,:frac13:[;;cup;;]:3,:+infty[ $$ ?







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closed as off-topic by Alex Francisco, amWhy, Jyrki Lahtonen, Cesareo, Parcly Taxel Jul 15 at 1:09


This question appears to be off-topic. The users who voted to close gave this specific reason:


  • "This question is missing context or other details: Please improve the question by providing additional context, which ideally includes your thoughts on the problem and any attempts you have made to solve it. This information helps others identify where you have difficulties and helps them write answers appropriate to your experience level." – Alex Francisco, amWhy, Jyrki Lahtonen, Cesareo, Parcly Taxel
If this question can be reworded to fit the rules in the help center, please edit the question.








  • 1




    Use cup to make the union symbol ($cup$) and iff to make the if and only if symbol ($iff$).
    – parsiad
    Jul 14 at 15:06










  • Thank you. Already edited.
    – Daniel Oscar
    Jul 14 at 15:13














up vote
0
down vote

favorite












Consider the following polynomial difference $A(x)-B(x)$:
$$A(x)=x^4-6x^3+8x^2+6x-9$$
$$B(x)=x^4-6x^2-8x-3$$



How can I prove/know that this difference satisfies this condition:
$$A(x)-B(x)<0 iff x:in :]-1,:frac13:[;;cup;;]:3,:+infty[ $$ ?







share|cite|improve this question













closed as off-topic by Alex Francisco, amWhy, Jyrki Lahtonen, Cesareo, Parcly Taxel Jul 15 at 1:09


This question appears to be off-topic. The users who voted to close gave this specific reason:


  • "This question is missing context or other details: Please improve the question by providing additional context, which ideally includes your thoughts on the problem and any attempts you have made to solve it. This information helps others identify where you have difficulties and helps them write answers appropriate to your experience level." – Alex Francisco, amWhy, Jyrki Lahtonen, Cesareo, Parcly Taxel
If this question can be reworded to fit the rules in the help center, please edit the question.








  • 1




    Use cup to make the union symbol ($cup$) and iff to make the if and only if symbol ($iff$).
    – parsiad
    Jul 14 at 15:06










  • Thank you. Already edited.
    – Daniel Oscar
    Jul 14 at 15:13












up vote
0
down vote

favorite









up vote
0
down vote

favorite











Consider the following polynomial difference $A(x)-B(x)$:
$$A(x)=x^4-6x^3+8x^2+6x-9$$
$$B(x)=x^4-6x^2-8x-3$$



How can I prove/know that this difference satisfies this condition:
$$A(x)-B(x)<0 iff x:in :]-1,:frac13:[;;cup;;]:3,:+infty[ $$ ?







share|cite|improve this question













Consider the following polynomial difference $A(x)-B(x)$:
$$A(x)=x^4-6x^3+8x^2+6x-9$$
$$B(x)=x^4-6x^2-8x-3$$



How can I prove/know that this difference satisfies this condition:
$$A(x)-B(x)<0 iff x:in :]-1,:frac13:[;;cup;;]:3,:+infty[ $$ ?









share|cite|improve this question












share|cite|improve this question




share|cite|improve this question








edited Jul 14 at 15:40









amWhy

189k25219431




189k25219431









asked Jul 14 at 15:04









Daniel Oscar

494




494




closed as off-topic by Alex Francisco, amWhy, Jyrki Lahtonen, Cesareo, Parcly Taxel Jul 15 at 1:09


This question appears to be off-topic. The users who voted to close gave this specific reason:


  • "This question is missing context or other details: Please improve the question by providing additional context, which ideally includes your thoughts on the problem and any attempts you have made to solve it. This information helps others identify where you have difficulties and helps them write answers appropriate to your experience level." – Alex Francisco, amWhy, Jyrki Lahtonen, Cesareo, Parcly Taxel
If this question can be reworded to fit the rules in the help center, please edit the question.




closed as off-topic by Alex Francisco, amWhy, Jyrki Lahtonen, Cesareo, Parcly Taxel Jul 15 at 1:09


This question appears to be off-topic. The users who voted to close gave this specific reason:


  • "This question is missing context or other details: Please improve the question by providing additional context, which ideally includes your thoughts on the problem and any attempts you have made to solve it. This information helps others identify where you have difficulties and helps them write answers appropriate to your experience level." – Alex Francisco, amWhy, Jyrki Lahtonen, Cesareo, Parcly Taxel
If this question can be reworded to fit the rules in the help center, please edit the question.







  • 1




    Use cup to make the union symbol ($cup$) and iff to make the if and only if symbol ($iff$).
    – parsiad
    Jul 14 at 15:06










  • Thank you. Already edited.
    – Daniel Oscar
    Jul 14 at 15:13












  • 1




    Use cup to make the union symbol ($cup$) and iff to make the if and only if symbol ($iff$).
    – parsiad
    Jul 14 at 15:06










  • Thank you. Already edited.
    – Daniel Oscar
    Jul 14 at 15:13







1




1




Use cup to make the union symbol ($cup$) and iff to make the if and only if symbol ($iff$).
– parsiad
Jul 14 at 15:06




Use cup to make the union symbol ($cup$) and iff to make the if and only if symbol ($iff$).
– parsiad
Jul 14 at 15:06












Thank you. Already edited.
– Daniel Oscar
Jul 14 at 15:13




Thank you. Already edited.
– Daniel Oscar
Jul 14 at 15:13










3 Answers
3






active

oldest

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up vote
1
down vote













HINT:



Note that $$A(x)-B(x)=-6x^3+14x^2+14x-6=-2(3x^3-7x^2-7x+3)$$



The polynomial in brackets is palindromic so $x=-1$ is clearly a root. Factoring gives $$3x^3-7x^2-7x+3=(x+1)(x-3)(3x-1)$$ so $$A(x)-B(x)=-6(x+1)(x-3)left(x-frac13right)$$



This is negative only when $$(x+1)(x-3)left(x-frac13right)$$ is positive.






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    up vote
    1
    down vote













    1) $x:in :]-1,:frac13:[cup ]:3,:+infty[$



    Means that if we divide the real numbers into four regions:



    $A = (-infty, -1]; B= (-1,frac 13);C=[frac 13, 3]; D = (3, infty)$



    If $x in A$ then $(x + 1) le 0; (x - frac 13) < 0; (x - 3) < 0$ and $(x+1)(x-frac 13)(x-3) le 0$ and $-6(x+1)(x-frac 13)(x-3) ge 0$.



    If $x in B$ then $(x + 1) > 0; (x -frac 13) < 0; (x-3)< 0$ and $(x+1)(x-frac 13)(x-3) > 0$ and $-6(x+1)(x-frac 13)(x-3) < 0$.



    If $x in C$ then $(x + 1) > 0; (x - frac 13) > 0; (x - 3)le 0$ and $(x+1)(x-frac 13)(x-3) le 0$ and $-6(x+1)(x-frac 13)(x-3) ge 0$.



    If $x in D$ then $(x + 1) > 0; (x -frac 13) > 0; (x-3)> 0$ and $(x+1)(x-frac 13)(x-3) > 0$ and $-6(x+1)(x-frac 13)(x-3) < 0$.



    So $-6(x+1)(x-frac 13)(x-3) = -6x^3 + 14x^2 + 14x -6=A(x) - B(x) < 0$ if and only if $x in B$ or $xin D$. Or in othere words $A(x) - B(x) < 0 iff x in A cup B$.



    2)



    $D(x) = A(x) - B(x) = -6x^3 +6x^2 + 8x^2 + 8x + 6x - 6$



    $= -6x^3 + 14x^2 + 14x - 6$



    $= -6x^3 - 6x^2 + 20x^2 + 20x - 6x -6$



    $=-6x^2(x+1) + 20x(x + 1) - 6(x+1)$



    $=-2(3x^2 - 10x + 3)(x+1)$



    $=-2(3x -1)(x -3)(x+1)$



    $= -6(x+1)(x-frac 13)(x-3)$.



    $D(x) < 0 $ when an odd number of $-6,x+1, x-frac 13, x-3$ are negative and the rest are positive. $-6$ is always negative we need an even number of $x+1, x-frac 13, x-3$ to be negative. In other words, none or two are negative.



    As $x-3 < x-frac 13 < x+1$ to have none of them negative means $x -3 ge 0$ or $x ge 3$.



    To have two of them negative is to have $x-3$ and $x - frac 13$ negative but $x + 1 > 0$. So $x-frac 13 < 0< x + 1$ or in other words $-1 < x < frac 13$.



    3) Use the $D(x)$ of part 2)



    Now $D(x) = 0$ if $x = -1, frac 13, $ or $3$ so those roots divide the real numbers into four regions and... continue as in part 1)






    share|cite|improve this answer




























      up vote
      0
      down vote













      Hint: the difference of your polynomials $$A(x),B(x)$$ can be written as $$-2, left( 3,x-1 right) left( x-3 right) left( x+1 right) $$






      share|cite|improve this answer




























        3 Answers
        3






        active

        oldest

        votes








        3 Answers
        3






        active

        oldest

        votes









        active

        oldest

        votes






        active

        oldest

        votes








        up vote
        1
        down vote













        HINT:



        Note that $$A(x)-B(x)=-6x^3+14x^2+14x-6=-2(3x^3-7x^2-7x+3)$$



        The polynomial in brackets is palindromic so $x=-1$ is clearly a root. Factoring gives $$3x^3-7x^2-7x+3=(x+1)(x-3)(3x-1)$$ so $$A(x)-B(x)=-6(x+1)(x-3)left(x-frac13right)$$



        This is negative only when $$(x+1)(x-3)left(x-frac13right)$$ is positive.






        share|cite|improve this answer

























          up vote
          1
          down vote













          HINT:



          Note that $$A(x)-B(x)=-6x^3+14x^2+14x-6=-2(3x^3-7x^2-7x+3)$$



          The polynomial in brackets is palindromic so $x=-1$ is clearly a root. Factoring gives $$3x^3-7x^2-7x+3=(x+1)(x-3)(3x-1)$$ so $$A(x)-B(x)=-6(x+1)(x-3)left(x-frac13right)$$



          This is negative only when $$(x+1)(x-3)left(x-frac13right)$$ is positive.






          share|cite|improve this answer























            up vote
            1
            down vote










            up vote
            1
            down vote









            HINT:



            Note that $$A(x)-B(x)=-6x^3+14x^2+14x-6=-2(3x^3-7x^2-7x+3)$$



            The polynomial in brackets is palindromic so $x=-1$ is clearly a root. Factoring gives $$3x^3-7x^2-7x+3=(x+1)(x-3)(3x-1)$$ so $$A(x)-B(x)=-6(x+1)(x-3)left(x-frac13right)$$



            This is negative only when $$(x+1)(x-3)left(x-frac13right)$$ is positive.






            share|cite|improve this answer













            HINT:



            Note that $$A(x)-B(x)=-6x^3+14x^2+14x-6=-2(3x^3-7x^2-7x+3)$$



            The polynomial in brackets is palindromic so $x=-1$ is clearly a root. Factoring gives $$3x^3-7x^2-7x+3=(x+1)(x-3)(3x-1)$$ so $$A(x)-B(x)=-6(x+1)(x-3)left(x-frac13right)$$



            This is negative only when $$(x+1)(x-3)left(x-frac13right)$$ is positive.







            share|cite|improve this answer













            share|cite|improve this answer



            share|cite|improve this answer











            answered Jul 14 at 15:16









            TheSimpliFire

            9,69261952




            9,69261952




















                up vote
                1
                down vote













                1) $x:in :]-1,:frac13:[cup ]:3,:+infty[$



                Means that if we divide the real numbers into four regions:



                $A = (-infty, -1]; B= (-1,frac 13);C=[frac 13, 3]; D = (3, infty)$



                If $x in A$ then $(x + 1) le 0; (x - frac 13) < 0; (x - 3) < 0$ and $(x+1)(x-frac 13)(x-3) le 0$ and $-6(x+1)(x-frac 13)(x-3) ge 0$.



                If $x in B$ then $(x + 1) > 0; (x -frac 13) < 0; (x-3)< 0$ and $(x+1)(x-frac 13)(x-3) > 0$ and $-6(x+1)(x-frac 13)(x-3) < 0$.



                If $x in C$ then $(x + 1) > 0; (x - frac 13) > 0; (x - 3)le 0$ and $(x+1)(x-frac 13)(x-3) le 0$ and $-6(x+1)(x-frac 13)(x-3) ge 0$.



                If $x in D$ then $(x + 1) > 0; (x -frac 13) > 0; (x-3)> 0$ and $(x+1)(x-frac 13)(x-3) > 0$ and $-6(x+1)(x-frac 13)(x-3) < 0$.



                So $-6(x+1)(x-frac 13)(x-3) = -6x^3 + 14x^2 + 14x -6=A(x) - B(x) < 0$ if and only if $x in B$ or $xin D$. Or in othere words $A(x) - B(x) < 0 iff x in A cup B$.



                2)



                $D(x) = A(x) - B(x) = -6x^3 +6x^2 + 8x^2 + 8x + 6x - 6$



                $= -6x^3 + 14x^2 + 14x - 6$



                $= -6x^3 - 6x^2 + 20x^2 + 20x - 6x -6$



                $=-6x^2(x+1) + 20x(x + 1) - 6(x+1)$



                $=-2(3x^2 - 10x + 3)(x+1)$



                $=-2(3x -1)(x -3)(x+1)$



                $= -6(x+1)(x-frac 13)(x-3)$.



                $D(x) < 0 $ when an odd number of $-6,x+1, x-frac 13, x-3$ are negative and the rest are positive. $-6$ is always negative we need an even number of $x+1, x-frac 13, x-3$ to be negative. In other words, none or two are negative.



                As $x-3 < x-frac 13 < x+1$ to have none of them negative means $x -3 ge 0$ or $x ge 3$.



                To have two of them negative is to have $x-3$ and $x - frac 13$ negative but $x + 1 > 0$. So $x-frac 13 < 0< x + 1$ or in other words $-1 < x < frac 13$.



                3) Use the $D(x)$ of part 2)



                Now $D(x) = 0$ if $x = -1, frac 13, $ or $3$ so those roots divide the real numbers into four regions and... continue as in part 1)






                share|cite|improve this answer

























                  up vote
                  1
                  down vote













                  1) $x:in :]-1,:frac13:[cup ]:3,:+infty[$



                  Means that if we divide the real numbers into four regions:



                  $A = (-infty, -1]; B= (-1,frac 13);C=[frac 13, 3]; D = (3, infty)$



                  If $x in A$ then $(x + 1) le 0; (x - frac 13) < 0; (x - 3) < 0$ and $(x+1)(x-frac 13)(x-3) le 0$ and $-6(x+1)(x-frac 13)(x-3) ge 0$.



                  If $x in B$ then $(x + 1) > 0; (x -frac 13) < 0; (x-3)< 0$ and $(x+1)(x-frac 13)(x-3) > 0$ and $-6(x+1)(x-frac 13)(x-3) < 0$.



                  If $x in C$ then $(x + 1) > 0; (x - frac 13) > 0; (x - 3)le 0$ and $(x+1)(x-frac 13)(x-3) le 0$ and $-6(x+1)(x-frac 13)(x-3) ge 0$.



                  If $x in D$ then $(x + 1) > 0; (x -frac 13) > 0; (x-3)> 0$ and $(x+1)(x-frac 13)(x-3) > 0$ and $-6(x+1)(x-frac 13)(x-3) < 0$.



                  So $-6(x+1)(x-frac 13)(x-3) = -6x^3 + 14x^2 + 14x -6=A(x) - B(x) < 0$ if and only if $x in B$ or $xin D$. Or in othere words $A(x) - B(x) < 0 iff x in A cup B$.



                  2)



                  $D(x) = A(x) - B(x) = -6x^3 +6x^2 + 8x^2 + 8x + 6x - 6$



                  $= -6x^3 + 14x^2 + 14x - 6$



                  $= -6x^3 - 6x^2 + 20x^2 + 20x - 6x -6$



                  $=-6x^2(x+1) + 20x(x + 1) - 6(x+1)$



                  $=-2(3x^2 - 10x + 3)(x+1)$



                  $=-2(3x -1)(x -3)(x+1)$



                  $= -6(x+1)(x-frac 13)(x-3)$.



                  $D(x) < 0 $ when an odd number of $-6,x+1, x-frac 13, x-3$ are negative and the rest are positive. $-6$ is always negative we need an even number of $x+1, x-frac 13, x-3$ to be negative. In other words, none or two are negative.



                  As $x-3 < x-frac 13 < x+1$ to have none of them negative means $x -3 ge 0$ or $x ge 3$.



                  To have two of them negative is to have $x-3$ and $x - frac 13$ negative but $x + 1 > 0$. So $x-frac 13 < 0< x + 1$ or in other words $-1 < x < frac 13$.



                  3) Use the $D(x)$ of part 2)



                  Now $D(x) = 0$ if $x = -1, frac 13, $ or $3$ so those roots divide the real numbers into four regions and... continue as in part 1)






                  share|cite|improve this answer























                    up vote
                    1
                    down vote










                    up vote
                    1
                    down vote









                    1) $x:in :]-1,:frac13:[cup ]:3,:+infty[$



                    Means that if we divide the real numbers into four regions:



                    $A = (-infty, -1]; B= (-1,frac 13);C=[frac 13, 3]; D = (3, infty)$



                    If $x in A$ then $(x + 1) le 0; (x - frac 13) < 0; (x - 3) < 0$ and $(x+1)(x-frac 13)(x-3) le 0$ and $-6(x+1)(x-frac 13)(x-3) ge 0$.



                    If $x in B$ then $(x + 1) > 0; (x -frac 13) < 0; (x-3)< 0$ and $(x+1)(x-frac 13)(x-3) > 0$ and $-6(x+1)(x-frac 13)(x-3) < 0$.



                    If $x in C$ then $(x + 1) > 0; (x - frac 13) > 0; (x - 3)le 0$ and $(x+1)(x-frac 13)(x-3) le 0$ and $-6(x+1)(x-frac 13)(x-3) ge 0$.



                    If $x in D$ then $(x + 1) > 0; (x -frac 13) > 0; (x-3)> 0$ and $(x+1)(x-frac 13)(x-3) > 0$ and $-6(x+1)(x-frac 13)(x-3) < 0$.



                    So $-6(x+1)(x-frac 13)(x-3) = -6x^3 + 14x^2 + 14x -6=A(x) - B(x) < 0$ if and only if $x in B$ or $xin D$. Or in othere words $A(x) - B(x) < 0 iff x in A cup B$.



                    2)



                    $D(x) = A(x) - B(x) = -6x^3 +6x^2 + 8x^2 + 8x + 6x - 6$



                    $= -6x^3 + 14x^2 + 14x - 6$



                    $= -6x^3 - 6x^2 + 20x^2 + 20x - 6x -6$



                    $=-6x^2(x+1) + 20x(x + 1) - 6(x+1)$



                    $=-2(3x^2 - 10x + 3)(x+1)$



                    $=-2(3x -1)(x -3)(x+1)$



                    $= -6(x+1)(x-frac 13)(x-3)$.



                    $D(x) < 0 $ when an odd number of $-6,x+1, x-frac 13, x-3$ are negative and the rest are positive. $-6$ is always negative we need an even number of $x+1, x-frac 13, x-3$ to be negative. In other words, none or two are negative.



                    As $x-3 < x-frac 13 < x+1$ to have none of them negative means $x -3 ge 0$ or $x ge 3$.



                    To have two of them negative is to have $x-3$ and $x - frac 13$ negative but $x + 1 > 0$. So $x-frac 13 < 0< x + 1$ or in other words $-1 < x < frac 13$.



                    3) Use the $D(x)$ of part 2)



                    Now $D(x) = 0$ if $x = -1, frac 13, $ or $3$ so those roots divide the real numbers into four regions and... continue as in part 1)






                    share|cite|improve this answer













                    1) $x:in :]-1,:frac13:[cup ]:3,:+infty[$



                    Means that if we divide the real numbers into four regions:



                    $A = (-infty, -1]; B= (-1,frac 13);C=[frac 13, 3]; D = (3, infty)$



                    If $x in A$ then $(x + 1) le 0; (x - frac 13) < 0; (x - 3) < 0$ and $(x+1)(x-frac 13)(x-3) le 0$ and $-6(x+1)(x-frac 13)(x-3) ge 0$.



                    If $x in B$ then $(x + 1) > 0; (x -frac 13) < 0; (x-3)< 0$ and $(x+1)(x-frac 13)(x-3) > 0$ and $-6(x+1)(x-frac 13)(x-3) < 0$.



                    If $x in C$ then $(x + 1) > 0; (x - frac 13) > 0; (x - 3)le 0$ and $(x+1)(x-frac 13)(x-3) le 0$ and $-6(x+1)(x-frac 13)(x-3) ge 0$.



                    If $x in D$ then $(x + 1) > 0; (x -frac 13) > 0; (x-3)> 0$ and $(x+1)(x-frac 13)(x-3) > 0$ and $-6(x+1)(x-frac 13)(x-3) < 0$.



                    So $-6(x+1)(x-frac 13)(x-3) = -6x^3 + 14x^2 + 14x -6=A(x) - B(x) < 0$ if and only if $x in B$ or $xin D$. Or in othere words $A(x) - B(x) < 0 iff x in A cup B$.



                    2)



                    $D(x) = A(x) - B(x) = -6x^3 +6x^2 + 8x^2 + 8x + 6x - 6$



                    $= -6x^3 + 14x^2 + 14x - 6$



                    $= -6x^3 - 6x^2 + 20x^2 + 20x - 6x -6$



                    $=-6x^2(x+1) + 20x(x + 1) - 6(x+1)$



                    $=-2(3x^2 - 10x + 3)(x+1)$



                    $=-2(3x -1)(x -3)(x+1)$



                    $= -6(x+1)(x-frac 13)(x-3)$.



                    $D(x) < 0 $ when an odd number of $-6,x+1, x-frac 13, x-3$ are negative and the rest are positive. $-6$ is always negative we need an even number of $x+1, x-frac 13, x-3$ to be negative. In other words, none or two are negative.



                    As $x-3 < x-frac 13 < x+1$ to have none of them negative means $x -3 ge 0$ or $x ge 3$.



                    To have two of them negative is to have $x-3$ and $x - frac 13$ negative but $x + 1 > 0$. So $x-frac 13 < 0< x + 1$ or in other words $-1 < x < frac 13$.



                    3) Use the $D(x)$ of part 2)



                    Now $D(x) = 0$ if $x = -1, frac 13, $ or $3$ so those roots divide the real numbers into four regions and... continue as in part 1)







                    share|cite|improve this answer













                    share|cite|improve this answer



                    share|cite|improve this answer











                    answered Jul 14 at 16:33









                    fleablood

                    60.5k22575




                    60.5k22575




















                        up vote
                        0
                        down vote













                        Hint: the difference of your polynomials $$A(x),B(x)$$ can be written as $$-2, left( 3,x-1 right) left( x-3 right) left( x+1 right) $$






                        share|cite|improve this answer

























                          up vote
                          0
                          down vote













                          Hint: the difference of your polynomials $$A(x),B(x)$$ can be written as $$-2, left( 3,x-1 right) left( x-3 right) left( x+1 right) $$






                          share|cite|improve this answer























                            up vote
                            0
                            down vote










                            up vote
                            0
                            down vote









                            Hint: the difference of your polynomials $$A(x),B(x)$$ can be written as $$-2, left( 3,x-1 right) left( x-3 right) left( x+1 right) $$






                            share|cite|improve this answer













                            Hint: the difference of your polynomials $$A(x),B(x)$$ can be written as $$-2, left( 3,x-1 right) left( x-3 right) left( x+1 right) $$







                            share|cite|improve this answer













                            share|cite|improve this answer



                            share|cite|improve this answer











                            answered Jul 14 at 15:08









                            Dr. Sonnhard Graubner

                            67k32660




                            67k32660












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