## Trigonometry (10th Edition)

1. Lets start by substituting values into expression: $\cos90^{\circ}+\sin90^{\circ}=0+1=1$ (correct) $\cos270^{\circ}+\sin270^{\circ}=0+(-1)=-1$(false) $\cos60^{\circ}+\sin60^{\circ}\ne1$ another counter example