若tana=2则sin^2a+sinacosa

cos(派/2+a)-2sin(a-派/2)=0
sin(-a)-2cos(π-a)=0
-sina+2cosa=0
sina=2cosa
tana=2
sin^2a+sinacosa-2cos^2a
=(sin^2a+sinacosa-2cos^2a)/(sin^2a+cos^2a)
=(tan^2a+tana-2)/(tan^2a+1)
=(4+2-2)/(4+1)
=4/5

若sina=2cosa,
sina/cosa=2;
则1/(sin^2a+sinacosa)
=1/(2sinacosa+sinacosa)
=(sin²a+cos²a)/(3sinacosa)
=(sin²a/cos²a+1)/(3sina/cosa)
=(2²+1)/(3×2)
=5/6;
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