Question

Cos( 90+∅ ) = - sin∅


prove it :-

Answer 1

According to ASTC rule
A-all trigonometric functions are possitive in 1st quadrant
S-sin and cosec function are possitive in 2nd quadrant
T-tan and cot function are possitive in 3rd quadrant
C-cos and sec function are possitive in 4th quadrant
quadrants means it implies to the 4 quadrants which are created by intersection of X and Y axes
so Cos(90+theta) is negative