please help with these problems thanks!

1.

cos^2(75degrees)

2.

sec^2(15degrees)

also

FInd the exact value given that cosx=1/3 with x in quad I and sinB= -1/2 with B in quad IV.

sin(x-B)

I got (2(squareroot)6-1)/6 but Im not 100% sure.

Find the exact value given that sinA= -4/5 with A in quad IV, tanB= 7/24 with B in quad III, and cosC= -5/13 with C in quad II.

Any help is gratefully appreciated!!!