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. b2 + c2 < a2

B. a2 + b2 = c2

C. a2 + c2 > b2

D. b2 + c2 > a2

Answer 1

theta is an obtuse angle if cos theta < 0

so the side a is the longest side in the triangle...

and the sum of the shorter sides must be greater than the length of the longest side to form a triangle.
b + c > a

this leads to

b^2 + c^2 < a^2