2/15

a) 3 + 7 + 11 + 15 + …. + 103
Dãy trên có: $\frac{103-3}{4}+1=26$ số hạng
3 + 7 + 11 + 15 + … + 103 = $\frac{(3+103)\times 26}{2}=1378$
b)
$\begin{align}
& \frac{2}{5\times 7}+\frac{2}{7\times 9}+\frac{2}{9\times 11}+\frac{2}{11\times 13}+\frac{2}{13\times 15} \\
& =\frac{7-5}{5\times 7}+\frac{9-7}{7\times 9}+\frac{11-9}{9\times 11}+\frac{13-11}{11\times 13}+\frac{15-13}{13\times 15} \\
& =\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+\frac{1}{9}-\frac{1}{11}+\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-\frac{1}{15} \\
& =\frac{1}{5}-\frac{1}{15}=\frac{3}{15}-\frac{1}{15}=\frac{2}{15} \\
\end{align}$