Question

Suppose a triangle has sides a, b, and c, and let theta be the angle opposite the side of length a. If cos theta < 0, what must be true?
A. a^2 + b^2 = c^2
B. a^2 + b^2 > c^2
C. b^2 + c^2 < a^2
D. b^2 + c^2 > a^2