Opposite-Angle Identities
Next, let’s look at the following set of identities.
Opposite-Angle Identities
sin(-A) = -sin A
cos(-A) = cos(A)
tan(-A) = -tan (A)
The Opposite-Angle Identities allow you to see what happens to trigonometric functions when you change the angle to it’s opposite.
For example, look at A = 45°.
tan(-A) = tan(-45°) = -1
and
-tan(45°) = -1