Question

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?

Answer 1

`c=sqrt(130-126cos(3/8pi)`

Explanation:

Use cosine theorem
`c^2=a^2+b^2-2abcos(3/8pi)`
So
`c^2=7^2+9^2-2*7*9cos(3/8pi)`
`c=sqrt(130-126cos(3/8pi)`