Question

How do you use the law of cosines or law of sines if you are given A = 120 degrees, b = 3, c = 10?

Answer 1

Use the trig identity: a^2 = b^2 + c^2 - 2bc.cos A
and sin A/a = sin B/b.

Explanation:

`a^2 = 9 + 100 - 2(3)(10)(-0.5) = 109 + 30 = 139 -> a = 11.80`

`sin B = (b.sin A)/a = (3(0.87))/11.80 = 2.61/11.80 = 0.22` ->

--> B = 12.78 deg
Angle C = 180 - A - B = 180 - 120 - 12.78 = 47.22