Question

[༄NguyễnTrungNghĩa༄༂]

Tinh nhanh :

\(\frac{1}{1.2.3}+\frac{1}{2.3.4}+\frac{1}{3.4.5}+...+\frac{1}{98.99.100}\)

Answer 1

[Trà My]

Đặt \(A=\frac{1}{1.2.3}+\frac{1}{2.3.4}+\frac{1}{3.4.5}+...+\frac{1}{98.99.100}\)

\(A=\frac{1}{2}\left(\frac{2}{1.2.3}+\frac{2}{2.3.4}+\frac{2}{3.4.5}+...+\frac{2}{98.99.100}\right)\)

\(A=\frac{1}{2}\left(\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+\frac{1}{3.4}-\frac{1}{4.5}+...+\frac{1}{98.99}-\frac{1}{99.100}\right)\)

\(A=\frac{1}{2}\left(\frac{1}{1.2}-\frac{1}{99.100}\right)\)

chỗ nãy rồi bạn tự tính tiếp

Answer 2

[༄NguyễnTrungNghĩa༄༂]

KQ la \(\frac{4949}{19800}\)ak cac ban