①已知cos(π/4-a)=3/5,sin(3π/

2个回答

1.因为(3π/4+b)-(π/4-a)=b+a+π/2
所以cos[（a+b）+π/2]=cos(a+b)*cosπ/2-sin(a+b)*sinπ/2=-sin(a+b)
所以sin(a+b)=-cos[（a+b）+π/2]=-cos[(3π/4+b)-(π/4-a)]
=-[cos(3π/4+b)*cos(π/4-a)+sin(3π/4+b)*sin(π/4-a)]
由π/4

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