Question

[Nguyễn Hương Giang]

Tính: 1/2 + 1/6 + 1/12 + 1/20 + 1/30 + .......... + 1/10100  (có 100 số hạng)

Answer 1

[Lục Minh Hoàng]

\(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+...+\frac{1}{10100}\)

\(=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+...+\frac{1}{100.101}\)

\(=\frac{2-1}{1.2}+\frac{3-2}{2.3}+\frac{4-3}{3.4}+\frac{5-4}{4.5}+\frac{6-5}{5.6}+...+\frac{101-100}{100.101}\)

\(=\frac{2}{1.2}-\frac{1}{1.2}+\frac{3}{2.3}-\frac{2}{2.3}+\frac{4}{3.4}-\frac{3}{3.4}+\frac{5}{4.5}-\frac{4}{4.5}+\frac{6}{5.6}-\frac{5}{6.5}+...+\frac{101}{100.101}-\frac{100}{100.101}\)

\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+...+\frac{1}{100}-\frac{1}{101}\)

\(=1-\frac{1}{101}\)

\(=\frac{100}{101}\)

Answer 2

[Hà Phương Trần Thị]

A= 1/1.2 +1/2.3 + 1/3.4 + 1/4.5 +...............+ 1/100.101

A= 1 - 1/2 +1/2 - 1/3 + 1/3 - 1/4 +..................+1/100 - 1/101

A= 1 - 1/101

A = 100/101