cos2a=cos^2a-1 怎么推
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cos2a=cos^2a-1 怎么推
cos2a=cos^2a-1 怎么推
cos2a=cos^2a-1 怎么推
cos2a=cos(a+a)
=cosacosa-sinasina
=cos²a-sin²a
=cos²a-(1-cos²a)
=2cos²a-1
(题目错了吧)