# A triangle has sides A, B, and C. Sides A and B are of lengths 7 and 9, respectively, and the angle between A and B is (3pi)/8 . What is the length of side C?

May 3, 2017

c=sqrt(130-126cos(3/8pi)
${c}^{2} = {a}^{2} + {b}^{2} - 2 a b \cos \left(\frac{3}{8} \pi\right)$
${c}^{2} = {7}^{2} + {9}^{2} - 2 \cdot 7 \cdot 9 \cos \left(\frac{3}{8} \pi\right)$
c=sqrt(130-126cos(3/8pi)