作业帮求证 sinx^2+siny^2-sinx^2siny^2+cosx^2cosy^2=1
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求证 sinx^2+siny^2-sinx^2siny^2+cosx^2cosy^2=1
cos(x+y)(x-y)
=(1/2)[cos(x+y+x-y)+cos(x+y-x+y)]
=(1/2)(cos2x+cos2y)
这步看不太懂
cos(x+y)(x-y)
=(1/2)[cos(x+y+x-y)+cos(x+y-x+y)]
=(1/2)(cos2x+cos2y)
这步看不太懂
sinx^2+siny^2-sinx^2siny^2+cosx^2cosy^2
=sinx^2(1-siny^2)+siny^2+cosx^2cosy^2
=sinx^2cosy^2+siny^2+cosx^2cosy^2
=cosy^2(sinx^2+cosy^2)+siny^2
=cosy^2+siny^2=1
=sinx^2(1-siny^2)+siny^2+cosx^2cosy^2
=sinx^2cosy^2+siny^2+cosx^2cosy^2
=cosy^2(sinx^2+cosy^2)+siny^2
=cosy^2+siny^2=1
已知sinx减siny等于负三分之二（sinx-siny=-2/3）；cosx加cosy等于三分之二（cosx+cosy
证明cosx(cosx-cosy)+sinx(sinx-siny)=2sin(x-y)/2
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