Lời giải
$C=\frac{1}{1.2.3}+\frac{1}{2.3.4}+...+\frac{1}{98.99.100}$
$2.C=\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+....+\frac{1}{98.99}-\frac{1}{99.100}$
$2.C=\frac{1}{2}-\frac{1}{99.100}$
$2.C=\frac{1}{2}-\frac{1}{9900}$
$C=\frac{4949}{9900.2}$
$C=\frac{4949}{19800}$