Answer:
Step-by-step explanation:
In the given figure the "hidden" vertex of the larger triangle is A.
Since, angle D is a right angle, both triangles BDC and CDE are right triangles (and they are also similar).
Since angle B and angle C have the same measure and angle D measures 90° (right angle), angle A and angle C are complementary, i.e.
For complementary angles, the sine of one acute angle equals the cosine of the other acute angle.
So, the sine of angle C is equal to the cosine of angle B, i.e. cos (B) = sin (A).