$\text{1 + 4 + 7 + ... + 94 + 97 + 100}$
$\text{Ta có: 1 + 100 = 101; 4 + 97 = 101; 7+ 94 = 101}$
$\text{Số số hạng là: (100 - 1) ÷ 3 + 1 = 34 (số hạng)}$
$\text{⇒ Có 17 cặp như thế}$
$\text{⇒ 1 + 4 + 7 + ... + 94 + 97 + 100 = 17 × 101 = 1717}$
$\text{⇒ 1717 ÷ a = 17 ⇒ a = 1717 ÷ 17 = 101}$
$\text{Vậy a = 101}$
$\text{A = 13.5 × 1420 + 45 × 780 × 3 ÷ 3 + 6 + 9 + ... + 24 + 27}$
$\text{= 13.5 × 1420 + 45 × 780 × 1 ÷ 90}$
$\text{= 13.5 × 1420 + 1 ÷ 2 × 780}$
$\text{= 1 ÷ 2 × 27 × 1420 + 1 ÷ 2 × 780}$
$\text{= 1 ÷ 2 × (27 × 1420 + 780) = 1 ÷ 2 × 2227 = 19 560}$
$\text{#nocopy}$
$\text{@bear}$
$\text{@team FA}$
