Pre Cal
posted by Lisa .
Sec^2 2u1/Sec^2 2u = Sin^2 2u

since all angles are 2u , let Ø  2u for ease of typing
I will assume you are proving this as an identity
LS = (1/cos^2 Ø  1)/(1/cos^2 Ø)
= (1  cos^2 Ø)/cos^2 Ø * (cos^2 Ø)
= 1  cos^2 Ø
= sin^2 Ø
= RS
replace Ø with 2u if you have to