sin(45+a)=根号5/5 则sin2a=?

sin(45+a)=根号5/5 则sin2a=? 在线等 最好过程详细~

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sin2a=-cos(90+2a)
=-[1-2sin^2(45+a)]
=-[1-2*1/5]
=-(3/5)
=-3/5

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