Trigonometry (11th Edition) Clone

$\sin{\left(\dfrac{\pi}{2}+\theta\right)}=\cos{\theta}$
Use a graphing utility to graph the given expression. (Refer to the graph below.) Note that the graph is identical to the graph of $\cos{\theta}$. This means that $\sin{(\frac{\pi}{2}+\theta)}=\cos{\theta}$. RECALL: $\sin{(A+B)}=\sin{A}\cos{B}+\cos{A}\sin{B}$ Use the identity above with $A=\frac{\pi}{2}$ and $B=\theta$ to obtain: \begin{align*} \sin{\left(\frac{\pi}{2}+\theta\right)}&=\sin{\frac{\pi}{2}}\cos{\theta}+\cos{\frac{\pi}{2}}\sin{\theta}\\\\ &=1\cdot \cos{\theta}+0\cdot\sin{\theta}\\\\ &=\cos{\theta}+0\\\\ &=\cos{\theta} \end{align*} Therefore, $\sin{\left(\dfrac{\pi}{2}+\theta\right)}=\cos{\theta}$