Question

A = 1/(1*2)+1/(2*3)+1/(3*4)+...+1/(18*19)+1/(19*20)

Answer 1

\(A=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+......+\frac{1}{18.19}+\frac{1}{19.20}\)

\(\Rightarrow A=\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{18}-\frac{1}{19}+\frac{1}{19}-\frac{1}{20}\)

\(\Rightarrow A=\frac{1}{1}-\frac{1}{20}\)

\(\Rightarrow A=\frac{19}{20}\)

Answer 2

\(A=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{18.19}\)

\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-...+\frac{1}{18}-\frac{1}{19}\)

\(=1-\frac{1}{19}=\frac{18}{19}\)