Normal
Gs n=x^2nếu x=2k->n=(2k)^2 chia hết cho 4nếu x=2k+1->n=(2k+1)^2 chia 4 dư 1
Gs n=x^2
nếu x=2k->n=(2k)^2 chia hết cho 4
nếu x=2k+1->n=(2k+1)^2 chia 4 dư 1
