Question

evaluate cos(tan^-1(-1)

Answer 1

Always draw a right triangle.

From the drawing, and considering the Quadrant and Reference Angle, we see that:

\(\tan^{-1}(-1) = -\dfrac{\pi}{4}\)

Again, considering the Quadrant and Reference Angle, we see that:
\(\cos(-\pi/4) = \dfrac{1}{\sqrt{2}}\)