已知cos(9π/4 -a)=根号2/4,则sin(2a+3π)的值为,要过程,谢谢.

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cos(9π/4 -a)
=cos(π/4 -a)
=根号2/2(cosa+sina)=根号2/4
则,cosa+sina=1/2
(cosa+sina)²=1+2sinacosa=1/4
2sinacosa=-3/4
sin(2a+3π)
= -sin(2a)
=-2sinacosa
=3/4