$\frac{1}{15}$ + $\frac{1}{35}$ + ... + $\frac{1}{2499}$
= $\frac{1}{2}$ . ($\frac{2}{3.5}$ + $\frac{2}{5.7}$ + ... + $\frac{2}{49.51}$)
= $\frac{1}{2}$ . (($\frac{1}{3}$ - $\frac{1}{5}$) + ($\frac{1}{5}$ - $\frac{1}{7}$) + ... + ($\frac{1}{49}$ - $\frac{1}{51}$))
= $\frac{1}{2}$ . ($\frac{1}{3}$ - $\frac{1}{5}$ + $\frac{1}{5}$ - $\frac{1}{7}$ + ... + $\frac{1}{49}$ - $\frac{1}{51}$)
= $\frac{1}{2}$ . ($\frac{1}{3}$ - $\frac{1}{51}$)
= $\frac{8}{51}$
Chúc học tốt!!!
