cos(75°+α)=1/3 则cos(150°-α)等于几.急……
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cos(75°+α)=1/3 则cos(150°-α)等于几.急……
cos(75°+α)=1/3,cos[45°+(30°+α)]=1/3
则有cos45°cos(30°+α)-sin45°sin(30°+α)=1/3 .(1)
cos(150°-α)=-cos[180°-(150°-α)]=-cos(30°+α) .(2)
将(1)式解开,得
根号2*{[(根号3)cosα-sinα]-[cosα+(根号3)sinα]}=4/3
根号2*[(根号3)-1](cosα-sinα)=4/3 .(3)
再根据cosα的平方+sinα的平方为1,综合(3)解出cosα、sinα的值并将其代入cos(30°+α)展开的式中,从而得出所求的值
则有cos45°cos(30°+α)-sin45°sin(30°+α)=1/3 .(1)
cos(150°-α)=-cos[180°-(150°-α)]=-cos(30°+α) .(2)
将(1)式解开,得
根号2*{[(根号3)cosα-sinα]-[cosα+(根号3)sinα]}=4/3
根号2*[(根号3)-1](cosα-sinα)=4/3 .(3)
再根据cosα的平方+sinα的平方为1,综合(3)解出cosα、sinα的值并将其代入cos(30°+α)展开的式中,从而得出所求的值
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