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ab sin(a+b)=-3/5 sin(b-π/4)=12/13 求cos(a+π/4))](/uploads/image/z/1689313-49-3.jpg?t=%E5%B7%B2%E7%9F%A5%E8%A7%92ab+sin%28a%2Bb%29%3D-3%2F5+sin%28b-%CF%80%2F4%29%3D12%2F13+%E6%B1%82cos%28a%2B%CF%80%2F4%29%29)
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