Question

已知tana=2，则sin^2*a+sinacosa-2cos^2*a=( )

Answer 1

tana=2，∴sina=2cosa ∴4cos^2*a+cos^2*a=1 ∴cos^2*a=1/5
∴sin^2*a+sinacosa-2cos^2*a
=4cos^2*a+2cos^2*a-2cos^2*a
=4cos^2*a=4/5

Answer 2

tana=2
sina/cosa=2
sina=2cosa
sina和cosa同号
(sina)^2+(cosa)^2=(2cosa)^2+(cosa)^2=1
(cosa)^2=1/5
(sina)^2=4/5
(sinacosa)^2=4/25
sinacosa=2/5

(sina)^2+sinacosa-2(cosa)^2
=4/5+2/5-2/5
=4/5

Answer 3

tana=2可得cos^2a=0.2
sin^2a+sinacosa-2cos^2a
=cos^2a(tan^2a+tana-2)
=0.8

Answer 4

ff