解
a,b∈(0,π/2)
∴cosa>0,sinb>0
∵sina=3/5
∴cosa=√1-(3/5)²=4/5
∵cosb=5/13
∴sinb=√1-(5/13)²=12/13
∴cos(a+b)
=cosacosb-sinasinb
=4/5×(5/13)-(3/5)(12/13)
=(20-36)/65
=-16/65