原来的式子=（sinasina-3sinacosa+4cosacosa)/(sinasina+cosacosa)=
(tanatana-3tana+4)/(tanatana+1) ...上下除以cosacoa
所以=(9-9+4)/(9+1)=2/5
2sin^2a-3sinacosa
=2*sina*tana*cosa-3sinacosa
=6sinacosa-3sinacosa
=3sinacosa
=3/2*sin(2a)
=3/2*((2*tan(a))/(1+tan^2(a)))
=3/2*2*3/(1+3^2)
=9/10
sin^2a-3sinacosa+4cos^2a
=(tana^2-3tana+4)/(1+tana^2)
tana=3 sina=3cosa ,
sin^2a-3sinacosa
=sin^2a-sin^2a
=0
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