tanx= -3/4
sinx/cosx= -3/4
sinx=(-3/4)cosx
(cosx)^2-(sinx)^2=(cosx)^2-[(-3/4)cosx]^2
=(cosx)^2-(9/16)(cosx)^2
=(7/16)(cosx)^2
=(7/16)/[1＋(tanx)^2]
=(7/16)/(1＋9/16)
=(7/16)/(25/16)
=7/25