Question

sin theta= 24/25 : find cos (2theta)

Answer 1

you need to find the cosine of theta first
do you know how to do that?

Answer 2

@satellite73 0<theta<pi/2

Answer 3

Alternatively, find theta = arc sin (24/25),
then get cos (2 theta). Assuming numerical answer is sought.

Answer 4

cos 2x = 1 - 2sin^2 x
sin x = 24/25 , -> sin^2 x = (24/25)^2 = (576/625)
cos 2x = 1 - 2(576/625) = -527/625 =-0.84