Question

Which relationship in the triangle must be true?

Answer 1

Cosine stands for "complementary sine."  The sine of an angle is the cosine of the complementary angle and vice versa.  Complementary angles add to 90°.

Answer: sin(B) = cos(90 - B)

It's true for any angle, and it doesn't have much particularly to do with a triangle.

Answer 2

COS(B)=sin(180-B).







Because a triangle have a 180 degree angle