sin（π/4+a）=3/5,
cos（π/2+2a）=cos2（π/4+a）=1-2sin^2（π/4+a）=1-2*9/25=7/25
sin2a=-cos（π/2+2a）=-7/25
已知sin（π/4+a）=3/5,则sin2a=
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