中等math-u07-d02
The cosine rule for the side a opposite angle A is:
A. a^2 = b^2 + c^2 - 2bc cos A✓ 答案
B. a^2 = b^2 + c^2 + 2bc cos A
C. a = b + c
D. a^2 = b^2 + c^2
解說
For a general triangle, a^2 = b^2 + c^2 - 2bc cos A. The version without the cos A term applies only when A is a right angle.