Tính nhẩm:

$\eqalign{ & 8 + 9 = ..... \cr  & 9 + 8 = ..... \cr  & 17 - 8 = ..... \cr  & 17 - 9 = ..... \cr}$       $\eqalign{& 5 + 7 = ..... \cr & 7 + 5 = ..... \cr & 12 - 5 = ..... \cr & 12 - 7 = ..... \cr}$

$\eqalign{ & 3 + 8 = ..... \cr  & 8 + 3 = ..... \cr  & 11 - 3 = ..... \cr  & 11 - 8 = ..... \cr}$        $\eqalign{& 4 + 9 = ..... \cr & 9 + 4 = ..... \cr & 13 - 4 = ..... \cr & 13 - 9 = ..... \cr}$

Số