解:∵sinα=1/3,α∈(0,π/2),∴cosa=根号(1-1/9)=2根号2/3∵cosβ=2/7,β是第四象限角∴sinb=-根号(1-4/49)=-3根号5/7∴cos(a-b)=cosacosb+sinasinb=2根号2/3*2/7+1/3*(-3根号5/7)=4根号2/21-根号5/7
