In triangle ABC, a=7.8, b=4.2, c=3.9. Find B

a. 15.1°
b. 148.7°
c. 78.9°
d. 16.2 °

It is D, right?

1 answer

Looks like a situation for the law of cosines.

b^2 = a^2 + c^2 - 2ac cos B
17.64 = 60.84 + 15.21 - 60.84 cos B
cos B = 0.9601
B = 16.2 degrees