已知角ab sin(a+b)=-3/5 sin(b-π/4)=12/13 求cos(a+π/4))

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已知角absin(a+b)=-3/5sin(b-π/4)=12/13求cos(a+π/4))已知角absin(a+b)=-3/5sin(b-π/4)=12/13求cos(a+π/4))已知角absin

已知角ab sin(a+b)=-3/5 sin(b-π/4)=12/13 求cos(a+π/4))
已知角ab sin(a+b)=-3/5 sin(b-π/4)=12/13 求cos(a+π/4))

已知角ab sin(a+b)=-3/5 sin(b-π/4)=12/13 求cos(a+π/4))
由已知可知cos(a+b)=-4/5,cos(b-π/4)=5/13;
则cos(a+π/4)=cos[(a+b)-(b-π/4)]=cos(a+b)cos(b-π/4)+sin(a+b)sin(b-π/4)=-56/65