# MPM2D Cosine Law – 2016-01-19

Yup, today was Cosine Law. If you have a triangle $ABC$ with opposite corresponding sides $abc$, you can find a missing side $c$ using

$c^2 = a^2 + b^2 - 2ab\cos(C)$

You need to know the lengths of $a$ and $b$ and the angle $C$ to do this.

You can also use the Cosine Law to find any missing angle if you know the lengths of all of the sides. In the above form, you can find angle $C$ by substituting the side lengths and rearranging.

Your homework tonight is to complete Page 409 #2 and Page 418 #2.