Đáp án:
$A$ = $\frac{18}{2.5}$ + $\frac{18}{5.8}$ + ... + $\frac{18}{203.206}$
$A$ = $\frac{6.3}{2.5}$ + $\frac{6.3}{5.8}$ + ... + $\frac{6.3}{203.206}$
$A$ = $6$ ( $\frac{3}{2.5}$ + $\frac{3}{5.8}$ + ... + $\frac{3}{203.206}$ )
$A$ = $6$ ( $\frac{1}{2}$ - $\frac{1}{5}$ + $\frac{1}{5}$ - $\frac{1}{8}$ + ... + $\frac{1}{203}$ - $\frac{1}{206}$ )
$A$ = $6$ ( $\frac{1}{2}$ - $\frac{1}{206}$ )
$A$ = $6$ . $\frac{51}{103}$
$A$ = $\frac{306}{103}$
chúc bạn học tốt ^^