sin=-3/5,-π/2

sin=-3/5,-π/2
sin=-3/5,-π/2

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

sin(2α)=2sinα·cosα
cos(2α)=cos^2;α-sin^2