sin=-3/5,-π/2
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sin=-3/5,-π/2sin=-3/5,-π/2sin=-3/5,-π/2因为sin=-3/5,-π/2所以cosa=4/5sin2a-cos2a=2sinacosa-（cos^2a-sin^2a
sin=-3/5,-π/2 sin=-3/5,-π/2 sin(2α)=2sinα·cosα
sin=-3/5,-π/2
因为sin=-3/5,-π/2
sin2a-cos2a=2sinacosa-（cos^2a-sin^2a）=2*(-3/5)*4/5-(16/25-9/25)=-31/25
cos(2α)=cos^2;α-sin^2