Question

已知tanX=2求2sin^2X+sinXcosX-3cos^2X

Answer 1

万能公式知道吧

是要记的哦...

sin2x=2tanx/(1+tan^2x)=4/5

cos2x=(1-tan^2x)/(1+tan^2x)=-3/5

原式=2sin^2x-1+1+sin2x/2-3(2cos^2x-1+1)/2

=-cos2x+1+sin2x/2-3cos2x/2-3/2

=-5cos2x/2+sin2x/2-1/2

=3/2+2/5-1/2

=7/5