Help me solve this question. Can't seem to grasp the formula

4)Given that sin 55degrees = 0.819 and cos20degrees = 0.94,

show how you would use the identities for sin(A+B) and cos(A+B) to find the following:

a) sin 75degrees

b) cos 35 degrees

c) tan 35 degrees

Showing the steps you would follow. NOTE: sin^2theta+cos^2theta=1