Question

question 12: standard g.srt.7
cos 72 degrees = sin ____ degrees
a. 18
b. 20
c. 12
d. 50

Explanation:

Step1: Recall co - function identity

We know the co - function identity in trigonometry: $\cos\theta=\sin(90^{\circ}-\theta)$.

Step2: Substitute $\theta = 72^{\circ}$

Substitute $\theta = 72^{\circ}$ into the identity $\cos\theta=\sin(90^{\circ}-\theta)$.
We get $\cos72^{\circ}=\sin(90 - 72)^{\circ}=\sin18^{\circ}$.

Answer:

a. 18