sina-cosa=1/2,求sin^3a-cos^3a值

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解：由sina-cosa=1/2等式两橡哪边平碰弯方有
（sina-cosa）^2=sin^2a+cos^2a+2sinacosa=1-2sinacosa=1/4
即sinacosa=3/8
又梁吵码sin^3a-cos^3a=（sina-cosa)(sin^2a+sinacosa+cos^2a）=1/2 * (1+3/8)=11/16

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