∵ a:b:c=3:4:5
∴ a:b:c=1/4:1/3:5/12
∴b=4/3a
c=5/3a
带入②得
a+4/3a+5/3a=36
4a=36
a=9
b=4/3a=4/3*9=12
c=5/3a=5/3*9=15