证明COS(X+Y)COS(X-Y)=COS^2X-SIN^2Y
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证明COS(X+Y)COS(X-Y)=COS^2X-SIN^2Y
证明COS(X+Y)COS(X-Y)=COS^2X-SIN^2Y
证明COS(X+Y)COS(X-Y)=COS^2X-SIN^2Y
COS(X+Y)COS(X-Y)
=(COSX*COSY-SINX*SINY)(COSX*COSY+SINX*SINY)
=(COSX*COSY)^2-(SINX*SINY)^2
=COS^2 X(1-SIN^2 Y)-(1-COS^2 X)SIN^2 Y
=COS^2 X -COS^2 X*SIN^2 Y-SIN^2 Y +COS^2 X SIN^2 Y
=COS^2X-SIN^2Y
COS(X+Y)COS(X-Y)
=1/2{cos【X+Y-(X-Y)】+cos【X+Y+(X-Y)】}
=1/2(cos2y+cos2x)
=1/2(2cos^2x-1+1-2sin^2y)
=COS^2X-SIN^2Y
证明COS(X+Y)COS(X-Y)=COS^2X-SIN^2Y
证明cos(x+y)cos(x-y)= cos^2(x)-sin^2(y)
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