# Math Help - Cosine law

1. ## Cosine law

Q : if $Cos B / a = Cos A / b$
Prove that : $a=b$ and $c^2$ $=$ $a^2$ $+$ $b^2$

2. ## Re: want help in cosine law

Originally Posted by mido22
Q : if $Cos B / a = Cos A / b$
Prove that : $a=b$ and $c^2$ $=$ $a^2$ $+$ $b^2$
What have you tried? You could start by using the cosine rule to express cos A and cos B in terms of a, b and c. Then try to simplify the resulting equation.

Edit. After checking the question, I think that you have stated it wrongly. It should say

Prove that : $a=b$ or $c^2=a^2+b^2$.

3. ## Re: want help in cosine law

yes it is or not and i made alot of tries but all reach to same result :
$b^2$ $($ $c^2$ $-$ $b^2$ $) =$ $a^2$ $($ $c^2$ $-$ $a^2$ $)$

4. ## Re: want help in cosine law

Originally Posted by mido22
yes it is or not and i made alot of tries but all reach to same result :
$b^2$ $($ $c^2$ $-$ $b^2$ $) =$ $a^2$ $($ $c^2$ $-$ $a^2$ $)$
Can you say a=b from this?

5. ## Re: want help in cosine law

no i can't say so becz a!=b

6. ## Re: want help in cosine law

Originally Posted by mido22
yes it is or not and i made alot of tries but all reach to same result :
$b^2$ $($ $c^2$ $-$ $b^2$ $) =$ $a^2$ $($ $c^2$ $-$ $a^2$ $)$
$b^2(c^2-b^2) = a^2(c^2-a^2)$

$b^2c^2 - b^4 = a^2c^2 - a^4$

$b^2c^2 - a^2c^2 = b^4 - a^4$

$c^2(b^2 - a^2) = (b^2 - a^2)(b^2 + a^2)$

since $a \ne b$ ...

$c^2 = b^2 + a^2$

7. ## Re: want help in cosine law

and how can i get a=b from same problem