If the sides of the Triangle are $a$, $b$ and $sqrt(a^2+ab+b^2)$, then find the value of the greatest Angle.

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If the sides of the Triangle are $a$, $b$ and $sqrt(a^2+ab+b^2)$, then find the value of the greatest Angle.



Answer Given in my notebook - $frac2pi3$ radians.



The Question is from the Chapter Solutions of Triangles and I'm aware of Sine and Cosine Rule of the triangles.




My try.



In any case, $a$ or $b$, no matter which side is greater, the 3rd side (Let's denote it by c) $ sqrt(a^2+ab+b^2) $ will have the greatest angle (i.e. greatest angle will be opposite to this side).



Now, let's take Opposite Angles of sides a, b and c as $A$, $B$, and $C$ respectively.
Then the only way to find the angle formed at angle C of the triangle can be easily calculated using Cosine Rule (if all the angles are given, which in this case is true), so putting the formula I get -



$$cos C = fraca^2+b^2-c^22ab = sqrt(a^2+ab+b^2)$$



Now, in the following case, I've to get the Value of Cosine function as $120^o or frac2pi3$ which I'm not sure how to do.



Can anyone help me with that?



Thanks :)







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  • 1




    You should have a $2$ in the denominator is your formula for the cosine.
    – saulspatz
    Jul 25 at 15:08










  • @saulspatz Thanks, but still it's difficult for me to solve
    – Abhas Kumar Sinha
    Jul 25 at 15:10






  • 1




    rather that the formual abouve, I think the law of cosines directly is more intiufitve. $c^2 = a^2 + b^2 - 2abcos C implies cos C = -frac 12$
    – Doug M
    Jul 25 at 15:27















up vote
0
down vote

favorite
1












If the sides of the Triangle are $a$, $b$ and $sqrt(a^2+ab+b^2)$, then find the value of the greatest Angle.



Answer Given in my notebook - $frac2pi3$ radians.



The Question is from the Chapter Solutions of Triangles and I'm aware of Sine and Cosine Rule of the triangles.




My try.



In any case, $a$ or $b$, no matter which side is greater, the 3rd side (Let's denote it by c) $ sqrt(a^2+ab+b^2) $ will have the greatest angle (i.e. greatest angle will be opposite to this side).



Now, let's take Opposite Angles of sides a, b and c as $A$, $B$, and $C$ respectively.
Then the only way to find the angle formed at angle C of the triangle can be easily calculated using Cosine Rule (if all the angles are given, which in this case is true), so putting the formula I get -



$$cos C = fraca^2+b^2-c^22ab = sqrt(a^2+ab+b^2)$$



Now, in the following case, I've to get the Value of Cosine function as $120^o or frac2pi3$ which I'm not sure how to do.



Can anyone help me with that?



Thanks :)







share|cite|improve this question

















  • 1




    You should have a $2$ in the denominator is your formula for the cosine.
    – saulspatz
    Jul 25 at 15:08










  • @saulspatz Thanks, but still it's difficult for me to solve
    – Abhas Kumar Sinha
    Jul 25 at 15:10






  • 1




    rather that the formual abouve, I think the law of cosines directly is more intiufitve. $c^2 = a^2 + b^2 - 2abcos C implies cos C = -frac 12$
    – Doug M
    Jul 25 at 15:27













up vote
0
down vote

favorite
1









up vote
0
down vote

favorite
1






1





If the sides of the Triangle are $a$, $b$ and $sqrt(a^2+ab+b^2)$, then find the value of the greatest Angle.



Answer Given in my notebook - $frac2pi3$ radians.



The Question is from the Chapter Solutions of Triangles and I'm aware of Sine and Cosine Rule of the triangles.




My try.



In any case, $a$ or $b$, no matter which side is greater, the 3rd side (Let's denote it by c) $ sqrt(a^2+ab+b^2) $ will have the greatest angle (i.e. greatest angle will be opposite to this side).



Now, let's take Opposite Angles of sides a, b and c as $A$, $B$, and $C$ respectively.
Then the only way to find the angle formed at angle C of the triangle can be easily calculated using Cosine Rule (if all the angles are given, which in this case is true), so putting the formula I get -



$$cos C = fraca^2+b^2-c^22ab = sqrt(a^2+ab+b^2)$$



Now, in the following case, I've to get the Value of Cosine function as $120^o or frac2pi3$ which I'm not sure how to do.



Can anyone help me with that?



Thanks :)







share|cite|improve this question













If the sides of the Triangle are $a$, $b$ and $sqrt(a^2+ab+b^2)$, then find the value of the greatest Angle.



Answer Given in my notebook - $frac2pi3$ radians.



The Question is from the Chapter Solutions of Triangles and I'm aware of Sine and Cosine Rule of the triangles.




My try.



In any case, $a$ or $b$, no matter which side is greater, the 3rd side (Let's denote it by c) $ sqrt(a^2+ab+b^2) $ will have the greatest angle (i.e. greatest angle will be opposite to this side).



Now, let's take Opposite Angles of sides a, b and c as $A$, $B$, and $C$ respectively.
Then the only way to find the angle formed at angle C of the triangle can be easily calculated using Cosine Rule (if all the angles are given, which in this case is true), so putting the formula I get -



$$cos C = fraca^2+b^2-c^22ab = sqrt(a^2+ab+b^2)$$



Now, in the following case, I've to get the Value of Cosine function as $120^o or frac2pi3$ which I'm not sure how to do.



Can anyone help me with that?



Thanks :)









share|cite|improve this question












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share|cite|improve this question








edited Jul 25 at 15:09
























asked Jul 25 at 15:05









Abhas Kumar Sinha

10011




10011







  • 1




    You should have a $2$ in the denominator is your formula for the cosine.
    – saulspatz
    Jul 25 at 15:08










  • @saulspatz Thanks, but still it's difficult for me to solve
    – Abhas Kumar Sinha
    Jul 25 at 15:10






  • 1




    rather that the formual abouve, I think the law of cosines directly is more intiufitve. $c^2 = a^2 + b^2 - 2abcos C implies cos C = -frac 12$
    – Doug M
    Jul 25 at 15:27













  • 1




    You should have a $2$ in the denominator is your formula for the cosine.
    – saulspatz
    Jul 25 at 15:08










  • @saulspatz Thanks, but still it's difficult for me to solve
    – Abhas Kumar Sinha
    Jul 25 at 15:10






  • 1




    rather that the formual abouve, I think the law of cosines directly is more intiufitve. $c^2 = a^2 + b^2 - 2abcos C implies cos C = -frac 12$
    – Doug M
    Jul 25 at 15:27








1




1




You should have a $2$ in the denominator is your formula for the cosine.
– saulspatz
Jul 25 at 15:08




You should have a $2$ in the denominator is your formula for the cosine.
– saulspatz
Jul 25 at 15:08












@saulspatz Thanks, but still it's difficult for me to solve
– Abhas Kumar Sinha
Jul 25 at 15:10




@saulspatz Thanks, but still it's difficult for me to solve
– Abhas Kumar Sinha
Jul 25 at 15:10




1




1




rather that the formual abouve, I think the law of cosines directly is more intiufitve. $c^2 = a^2 + b^2 - 2abcos C implies cos C = -frac 12$
– Doug M
Jul 25 at 15:27





rather that the formual abouve, I think the law of cosines directly is more intiufitve. $c^2 = a^2 + b^2 - 2abcos C implies cos C = -frac 12$
– Doug M
Jul 25 at 15:27











2 Answers
2






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up vote
2
down vote













$$cos C=fraca^2+b^2-c^22abtag1$$
as you nearly say, but $c$ is $sqrta^2+ab+b^2$. Substituting that
into $(1)$ should give you the $C$ that you want.






share|cite|improve this answer





















  • That's what I'm unable to do, How can you solve such equations?
    – Abhas Kumar Sinha
    Jul 25 at 15:09






  • 1




    No need to solve: substitute!
    – Lord Shark the Unknown
    Jul 25 at 15:09










  • Substitute what in where?
    – Abhas Kumar Sinha
    Jul 25 at 15:10






  • 1




    Substitute $sqrta^2+ab+b^2$ for $c$
    – saulspatz
    Jul 25 at 15:11










  • oh okay, I followed it, by mistake, I substituted that size side c, in angle C of cos..... My bad XD :)
    – Abhas Kumar Sinha
    Jul 25 at 15:11

















up vote
0
down vote













In Triangle ABC,



the side $c=sqrta^2+ab+b^2$ ----------------------(1)



and by cosine Rule, -



$cos C = fraca^2+b^2-c^22ab$ ------------------------------(2)



So, substituting (1) in (2) gives,



$$cos C = fraca^2+b^2-(a^2+ab+b^2)2ab$$
$$Rightarrow cos C = frac-ab2ab$$
$$iff C = 2npi pm frac2pi3$$ (for $n$ as any integer)



The rule of the Sum angle Property only stands correct only when $n=1$, thus $120^o$ or $frac2pi3$ is the reqd. answer






share|cite|improve this answer





















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    2 Answers
    2






    active

    oldest

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    2 Answers
    2






    active

    oldest

    votes









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    oldest

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    active

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    up vote
    2
    down vote













    $$cos C=fraca^2+b^2-c^22abtag1$$
    as you nearly say, but $c$ is $sqrta^2+ab+b^2$. Substituting that
    into $(1)$ should give you the $C$ that you want.






    share|cite|improve this answer





















    • That's what I'm unable to do, How can you solve such equations?
      – Abhas Kumar Sinha
      Jul 25 at 15:09






    • 1




      No need to solve: substitute!
      – Lord Shark the Unknown
      Jul 25 at 15:09










    • Substitute what in where?
      – Abhas Kumar Sinha
      Jul 25 at 15:10






    • 1




      Substitute $sqrta^2+ab+b^2$ for $c$
      – saulspatz
      Jul 25 at 15:11










    • oh okay, I followed it, by mistake, I substituted that size side c, in angle C of cos..... My bad XD :)
      – Abhas Kumar Sinha
      Jul 25 at 15:11














    up vote
    2
    down vote













    $$cos C=fraca^2+b^2-c^22abtag1$$
    as you nearly say, but $c$ is $sqrta^2+ab+b^2$. Substituting that
    into $(1)$ should give you the $C$ that you want.






    share|cite|improve this answer





















    • That's what I'm unable to do, How can you solve such equations?
      – Abhas Kumar Sinha
      Jul 25 at 15:09






    • 1




      No need to solve: substitute!
      – Lord Shark the Unknown
      Jul 25 at 15:09










    • Substitute what in where?
      – Abhas Kumar Sinha
      Jul 25 at 15:10






    • 1




      Substitute $sqrta^2+ab+b^2$ for $c$
      – saulspatz
      Jul 25 at 15:11










    • oh okay, I followed it, by mistake, I substituted that size side c, in angle C of cos..... My bad XD :)
      – Abhas Kumar Sinha
      Jul 25 at 15:11












    up vote
    2
    down vote










    up vote
    2
    down vote









    $$cos C=fraca^2+b^2-c^22abtag1$$
    as you nearly say, but $c$ is $sqrta^2+ab+b^2$. Substituting that
    into $(1)$ should give you the $C$ that you want.






    share|cite|improve this answer













    $$cos C=fraca^2+b^2-c^22abtag1$$
    as you nearly say, but $c$ is $sqrta^2+ab+b^2$. Substituting that
    into $(1)$ should give you the $C$ that you want.







    share|cite|improve this answer













    share|cite|improve this answer



    share|cite|improve this answer











    answered Jul 25 at 15:08









    Lord Shark the Unknown

    84.9k950111




    84.9k950111











    • That's what I'm unable to do, How can you solve such equations?
      – Abhas Kumar Sinha
      Jul 25 at 15:09






    • 1




      No need to solve: substitute!
      – Lord Shark the Unknown
      Jul 25 at 15:09










    • Substitute what in where?
      – Abhas Kumar Sinha
      Jul 25 at 15:10






    • 1




      Substitute $sqrta^2+ab+b^2$ for $c$
      – saulspatz
      Jul 25 at 15:11










    • oh okay, I followed it, by mistake, I substituted that size side c, in angle C of cos..... My bad XD :)
      – Abhas Kumar Sinha
      Jul 25 at 15:11
















    • That's what I'm unable to do, How can you solve such equations?
      – Abhas Kumar Sinha
      Jul 25 at 15:09






    • 1




      No need to solve: substitute!
      – Lord Shark the Unknown
      Jul 25 at 15:09










    • Substitute what in where?
      – Abhas Kumar Sinha
      Jul 25 at 15:10






    • 1




      Substitute $sqrta^2+ab+b^2$ for $c$
      – saulspatz
      Jul 25 at 15:11










    • oh okay, I followed it, by mistake, I substituted that size side c, in angle C of cos..... My bad XD :)
      – Abhas Kumar Sinha
      Jul 25 at 15:11















    That's what I'm unable to do, How can you solve such equations?
    – Abhas Kumar Sinha
    Jul 25 at 15:09




    That's what I'm unable to do, How can you solve such equations?
    – Abhas Kumar Sinha
    Jul 25 at 15:09




    1




    1




    No need to solve: substitute!
    – Lord Shark the Unknown
    Jul 25 at 15:09




    No need to solve: substitute!
    – Lord Shark the Unknown
    Jul 25 at 15:09












    Substitute what in where?
    – Abhas Kumar Sinha
    Jul 25 at 15:10




    Substitute what in where?
    – Abhas Kumar Sinha
    Jul 25 at 15:10




    1




    1




    Substitute $sqrta^2+ab+b^2$ for $c$
    – saulspatz
    Jul 25 at 15:11




    Substitute $sqrta^2+ab+b^2$ for $c$
    – saulspatz
    Jul 25 at 15:11












    oh okay, I followed it, by mistake, I substituted that size side c, in angle C of cos..... My bad XD :)
    – Abhas Kumar Sinha
    Jul 25 at 15:11




    oh okay, I followed it, by mistake, I substituted that size side c, in angle C of cos..... My bad XD :)
    – Abhas Kumar Sinha
    Jul 25 at 15:11










    up vote
    0
    down vote













    In Triangle ABC,



    the side $c=sqrta^2+ab+b^2$ ----------------------(1)



    and by cosine Rule, -



    $cos C = fraca^2+b^2-c^22ab$ ------------------------------(2)



    So, substituting (1) in (2) gives,



    $$cos C = fraca^2+b^2-(a^2+ab+b^2)2ab$$
    $$Rightarrow cos C = frac-ab2ab$$
    $$iff C = 2npi pm frac2pi3$$ (for $n$ as any integer)



    The rule of the Sum angle Property only stands correct only when $n=1$, thus $120^o$ or $frac2pi3$ is the reqd. answer






    share|cite|improve this answer

























      up vote
      0
      down vote













      In Triangle ABC,



      the side $c=sqrta^2+ab+b^2$ ----------------------(1)



      and by cosine Rule, -



      $cos C = fraca^2+b^2-c^22ab$ ------------------------------(2)



      So, substituting (1) in (2) gives,



      $$cos C = fraca^2+b^2-(a^2+ab+b^2)2ab$$
      $$Rightarrow cos C = frac-ab2ab$$
      $$iff C = 2npi pm frac2pi3$$ (for $n$ as any integer)



      The rule of the Sum angle Property only stands correct only when $n=1$, thus $120^o$ or $frac2pi3$ is the reqd. answer






      share|cite|improve this answer























        up vote
        0
        down vote










        up vote
        0
        down vote









        In Triangle ABC,



        the side $c=sqrta^2+ab+b^2$ ----------------------(1)



        and by cosine Rule, -



        $cos C = fraca^2+b^2-c^22ab$ ------------------------------(2)



        So, substituting (1) in (2) gives,



        $$cos C = fraca^2+b^2-(a^2+ab+b^2)2ab$$
        $$Rightarrow cos C = frac-ab2ab$$
        $$iff C = 2npi pm frac2pi3$$ (for $n$ as any integer)



        The rule of the Sum angle Property only stands correct only when $n=1$, thus $120^o$ or $frac2pi3$ is the reqd. answer






        share|cite|improve this answer













        In Triangle ABC,



        the side $c=sqrta^2+ab+b^2$ ----------------------(1)



        and by cosine Rule, -



        $cos C = fraca^2+b^2-c^22ab$ ------------------------------(2)



        So, substituting (1) in (2) gives,



        $$cos C = fraca^2+b^2-(a^2+ab+b^2)2ab$$
        $$Rightarrow cos C = frac-ab2ab$$
        $$iff C = 2npi pm frac2pi3$$ (for $n$ as any integer)



        The rule of the Sum angle Property only stands correct only when $n=1$, thus $120^o$ or $frac2pi3$ is the reqd. answer







        share|cite|improve this answer













        share|cite|improve this answer



        share|cite|improve this answer











        answered Jul 25 at 15:26









        Abhas Kumar Sinha

        10011




        10011






















             

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