1991

Than IQ lam dai qua!

to cung cong nhan bai nay don gian that chi can bien doi mot ti la ra (b^2 chia het cho 10 suy ra b=0)

Ta can gioi han gia tri cua x bang cach dung BDT sau do dat nhan tu o ve trai .Cuoi cung tim duoc nhung gia tri cua x la 13,14,20,21 va tinh ra duoc so co bon chu so can tim