# Trig Identities

• October 23rd 2010, 01:22 PM
GAVREED2
Trig Identities
Hello could someone please solve this question for me. I need to see the steps invloved as i am not really sure of what i am meant to be doing. Thankyou so somuch (Talking)

Question

Using the identity sin^2theta + cos^2theta=1 and the identities for cos(A+B) find:
cos 75degrees if cos 45degrees = .707, and cos 30degrees = 0.866
Trig
• October 23rd 2010, 02:32 PM
emakarov
You know $\cos 45^\circ$ and $\cos 30^\circ$. From the identity $\cos^2\alpha+\sin^2\alpha=1$, you also know $\sin 45^\circ$ and $\sin 30^\circ$. Maybe $75^\circ$ can be represented using $45^\circ$ and $30^\circ$ so that you can compute $\cos 75^\circ$ using the info above?