(1)a=pai/3
sina=sin(pai/3)=3^1/2/2
(2)tan(a-b)=(tana-tanb)/(1+tanatanb)
=(3-3/4)/(1+3*3/4)=(9/4)/(13/4)=9/13
(3)y=1-6sinx-2(1-sin^2x)
y=1-6sinx-2+2sin^2x
y=2sin^2x-6sinx-1
-1<=t=sinx<=1
y=2t^2-6t-1
y=2(t^2-3t-1/2)
y=2((t-3/2)^2-9/4-1/2)
y=2((t-3/2)^2-11/4)
y=2(t-3/2)^2-11/2
t=1,ymin=2-6-1=-4-1=3
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