∵sina=2/3,a属于(π/2,π)
∴cosa=-√5/3
∵cosb=-3/4,b属于(π,3π/2)
∴sinb=-√7/4
∴cos(a+b)=cosacosb-sinasinb
=(-√5/3)*(-3/4)-(2/3)*(-√7/4)
=(3√5+2√7)/12
cos(a-b)=cosacosb+sinasinb
=(-√5/3)*(-3/4)+(2/3)*(-√7/4)
=(3√5-2√7)/12