a) 10 + 12 + 14 + ... + 198 + 200 ( Có 96 số hạng )
= ( 10 + 200 ) x 96 : 2 = 10080
b) \(\frac{1}{5\times7}+\frac{1}{7\times9}+\frac{1}{2011\times2013}\)
\(=\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{2011}-\frac{1}{2013}\)
\(=\frac{1}{5}-\frac{1}{2013}\)
\(=\frac{2008}{10065}\)