cosα等于-4/5,α是第三项的角求sin(3π/4-α),cos(π/4+2α)的值

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cosα等于-4/5,α是第三项的角求sin(3π/4-α),cos(π/4+2α)的值
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cosα等于-4/5,α是第三项的角求sin(3π/4-α),cos(π/4+2α)的值
cosα等于-4/5,α是第三项的角求sin(3π/4-α),cos(π/4+2α)的值

cosα等于-4/5,α是第三项的角求sin(3π/4-α),cos(π/4+2α)的值
cosα=-4/5,α是第三象限角
sinα=-√(1-cos^2α)=-3/5
sin(3π/4-α)=sin3π/4cosα-cos3π/4sinα=-7√2/10
cos(π/4+2α)=cosπ/4cos2α-sinπ/4sin2α=√2/2(cos^2α-sin^2α-2sinαcosα)=-7√2//50

(1)-3+4倍的根号3/10
(2)7倍的根号2/25

sin(3π/4-α)= -7√2/10