Bạn làm đúng rồi nha.
A = $\frac{1}{1.2}$ + $\frac{1}{2.3}$ + ... + $\frac{1}{49.50}$
Ta thấy : $\frac{1}{1.2}$ = 1 - $\frac{1}{2}$
$\frac{1}{2.3}$ = $\frac{1}{2}$ - $\frac{1}{3}$
......................
$\frac{1}{49.50}$ = $\frac{1}{49}$ - $\frac{1}{50}$
=> A = 1 - $\frac{1}{2}$ + $\frac{1}{2}$ - $\frac{1}{3}$ + ... + $\frac{1}{49}$ - $\frac{1}{50}$
=> A = 1 - $\frac{1}{50}$
=> A = $\frac{50}{50}$ - $\frac{1}{50}$
=> A = $\frac{49}{50}$.
