sin a=1/3 则sin 3a=?
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sin(3a)=sin(2a+a)

=sin(2a)cosa+cos(2a)sina

=2sinacosacosa+[1-2(sina)^2]sina

=2sina[1-(sina)^2]+sina-2(sina)^3

=2sina-2(sina)^3+sina-2(sina)^3

=3sina-4(sina)^3

=3(1/3)-4(1/3)^3

=1-4/27

=23/27

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