1)
a = 7
$a^2=7^2=49$
2)
$a^2=64$
$a^2=8^2$
a = 8
3)
a = 12
$a^2={12}^2=144$
4)
$a^2=400$
$a^2={20}^2$
a = 20
5)
a = 50
$a^2={50}^2=2500$
6)
$a^2=10000$
$a^2={100}^2$
a = 100
7)
a = 0
$a^2=0^2=0$