# Thread: Law of cosine and sine

1. ## Law of cosine and sine

Hi,

The three sides of the triangle being a, b and c.

1. Can anyone get me the derivation of the law of cosine.
$\displaystyle c=a^2+b^2-(2ab cos \theta)$

2. Also, the law of sine.

$\displaystyle (sin A)/a = (sin B)/b = (sin C)/c$

where,
A, B and C are the internal angles subtended by the sides of the triangle a, b and c respectively.

2. Originally Posted by willki
Hi,

The three sides of the triangle being a, b and c.

1. Can anyone get me the derivation of the law of cosine.
$\displaystyle c=a^2+b^2-(2ab cos \theta)$

2. Also, the law of sine.

$\displaystyle (sin A)/a = (sin B)/b = (sin C)/c$

where,
A, B and C are the internal angles subtended by the sides of the triangle a, b and c respectively.
You'll find six proofs of the cosine rule here, and a discussion of the sine rule here.