证明题:cos3a =4cos³a-3cosa
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证明题:cos3a =4cos³a-3cosa
证明题:cos3a =4cos³a-3cosa
证明题:cos3a =4cos³a-3cosa
cos3a=cos(2a+a)
=cos2acosa-sin2asina
=(cos^2 a-sin^2a)cosa -2sin^2 acosa
=cosa( cos^2 a-sin^2 a-2sin^2 a)
=4cos^3 a-3cosa