Find the exact functional value If cos C 513 and C termina

Find the exact functional value.
If cos C = - 5/13, and C terminates in quadrant 2, then find cos 2C

If cos 2theta = -24/25, and pie/2 < 2theta < pie, then find sintheta

cosC = -5/13

C is in QII

cos2C = 2cos^2(C) - 1

= 2(-5/13)^2 -1 = 50/169 - 1

= - 129/169

cos2theta = -24/25

cos2theta = 1- 2sin^2theta

2sin^2theta = 1 - cos2theta

sintheta = sqrt[( 1- cos2theta)/2]

= sqrt[(1 +24/25)/2]

= sqrt[49/50]

= 7/5sqrt2