Question

Tính tổng\(A=\frac{1}{2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}\)

Answer 1

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

= 1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5

= 1 - 1/5

= 5/5 - 1/5

= 4/5