已知tana=2,求sin^2a+3sina cosa 的值.为什么sin^2a=4/5 ?

问题描述:

已知tana=2,求sin^2a+3sina cosa 的值.
为什么sin^2a=4/5 ?

tana=sina/cosa=2
sin^2a=4cos^2a=4(1-sin^2a)
sin^2a=4/5
sin^2a+3sina cosa
=sin^2a(1+3/tana)
=4/5*(1+3/2)
=2