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
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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
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)

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