help for beginer

[libed]

I made this program, but it doesn't work like I want.:

a = int(0)
for a in range (11,21):
    r = '({},{},{})'.format(a, a ** 2, a ** 3)
print(r)

I'd like to have squares and cubes numbers from 11 to 20. However, it shows only for number 20. Where is my mistake please?