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(Cos2x)/4+3/4是如何转变成(1+Cos4x)/8+3/4的?请...

解:∵sin(x-3π/4)cos(x-π/4)=-1/4 ==>[sin((x-3π/4)+(x-π/4))+sin((x-3π/4)-(x-π/4))]/2=-1/4 (应用正弦积化和差公式) ==>sin(2x-π)+sin(-π/2)=-1/2 ==>-sin(2x)-1=-1/2 ==>sin(2x)=-1/2 ∴cos(4x)=1-2sin²(2x) (应用余弦倍角公式) =1-2(-...

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