a//b
=> sinx/(3/4) = cosx/(-1)
-4sinx=3cosx
tanx = -3/4
(cosx)^2 = (4/5)^2 = 16/25
(cosx)^2 - (sinx)^2
= 2(cosx)^2-1
=2(16/25)-1
=7/25