tanx=2 求sin^2x+sinxcosx-3cos^2x
问题描述：
tanx=2 求sin^2x+sinxcosx-3cos^2x
答
tanx=2
sinx/cosx=2
sinx=2cosx
(sinx)^2+(cosx)^2=5(cosx)^2=1
(cosx)^2=1/5
(sinx)^2+sinxcosx-3(cosx)^2
=(cosx)^2(4+2-3)
=3(cosx)^2
=3/5