46.15.已知αβ∈(3π/4,π),sin(α+β)=-3/5 sin(β-π/4)=12/13,则cos(α+π/4)= .-56/65
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![46.15.已知αβ∈(3π/4,π),sin(α+β)=-3/5 sin(β-π/4)=12/13,则cos(α+π/4)= .-56/65](/uploads/image/z/1035913-49-3.jpg?t=46.15%EF%BC%8E%E5%B7%B2%E7%9F%A5%CE%B1%CE%B2%E2%88%88%283%CF%80%2F4%2C%CF%80%29%2Csin%28%CE%B1%2B%CE%B2%29%3D%EF%BC%8D3%2F5+sin%28%CE%B2-%CF%80%2F4%29%3D12%2F13%2C%E5%88%99cos%28%CE%B1%2B%CF%80%2F4%29%3D+.-56%2F65)
46.15.已知αβ∈(3π/4,π),sin(α+β)=-3/5 sin(β-π/4)=12/13,则cos(α+π/4)= .-56/65
46.15.已知αβ∈(3π/4,π),sin(α+β)=-3/5 sin(β-π/4)=12/13,则cos(α+π/4)= .-56/65
46.15.已知αβ∈(3π/4,π),sin(α+β)=-3/5 sin(β-π/4)=12/13,则cos(α+π/4)= .-56/65
∵α,β∈(3π/4,π)
∴α+β∈(3π/2,2π)
∴β-π/4∈(2π/4,3π/4)
∴cos(a+b)>0
cos(b-π/4)