a∈(π/4,π/2),b∈(0,π/4),cos(2a-b)=-11/14,sin(a-2b)=4√3/7,则cos(a+b)=---------

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a∈(π/4,π/2),b∈(0,π/4),cos(2a-b)=-11/14,sin(a-2b)=4√3/7,则cos(a+b)=---------

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