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:
$$A(x)-B(x)<0 iff x:in :]-1,:frac13:[;;cup;;]:3,:+infty[ $$ ?
elementary-set-theory polynomials
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
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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:
$$A(x)-B(x)<0 iff x:in :]-1,:frac13:[;;cup;;]:3,:+infty[ $$ ?
elementary-set-theory polynomials
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
1
Usecup
to make the union symbol ($cup$) andiff
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
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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:
$$A(x)-B(x)<0 iff x:in :]-1,:frac13:[;;cup;;]:3,:+infty[ $$ ?
elementary-set-theory polynomials
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:
$$A(x)-B(x)<0 iff x:in :]-1,:frac13:[;;cup;;]:3,:+infty[ $$ ?
elementary-set-theory polynomials
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
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
1
Usecup
to make the union symbol ($cup$) andiff
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
add a comment |Â
1
Usecup
to make the union symbol ($cup$) andiff
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
add a comment |Â
3 Answers
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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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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)
add a comment |Â
up vote
0
down vote
Hint: the difference of your polynomials $$A(x),B(x)$$ can be written as $$-2, left( 3,x-1 right) left( x-3 right) left( x+1 right) $$
add a comment |Â
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.
add a comment |Â
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.
add a comment |Â
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.
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.
answered Jul 14 at 15:16
TheSimpliFire
9,69261952
9,69261952
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up vote
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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)
add a comment |Â
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)
add a comment |Â
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)
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)
answered Jul 14 at 16:33
fleablood
60.5k22575
60.5k22575
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up vote
0
down vote
Hint: the difference of your polynomials $$A(x),B(x)$$ can be written as $$-2, left( 3,x-1 right) left( x-3 right) left( x+1 right) $$
add a comment |Â
up vote
0
down vote
Hint: the difference of your polynomials $$A(x),B(x)$$ can be written as $$-2, left( 3,x-1 right) left( x-3 right) left( x+1 right) $$
add a comment |Â
up vote
0
down vote
up vote
0
down vote
Hint: the difference of your polynomials $$A(x),B(x)$$ can be written as $$-2, left( 3,x-1 right) left( x-3 right) left( x+1 right) $$
Hint: the difference of your polynomials $$A(x),B(x)$$ can be written as $$-2, left( 3,x-1 right) left( x-3 right) left( x+1 right) $$
answered Jul 14 at 15:08


Dr. Sonnhard Graubner
67k32660
67k32660
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1
Use
cup
to make the union symbol ($cup$) andiff
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