(3π/4 +y)-(π/4 -x)
=π/2+(x+y)
所以
sin(x+y)=-cos[π/2+(x+y)]
=-cos[(3π/4 +y)-(π/4 -x)]
令A=(3π/4 +y),B=(π/4 -x)
π/2
已知cos(π/4 -x)=1/7,sin(3π/4 +y)=11/14,π/4
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