Question

N=\(\frac{1}{1.2}\)+ \(\frac{1}{2.3}\)+\(\frac{1}{3.4}\)+... +\(\frac{1}{a\left(a+1\right)}\)

 

Answer 1

Thay n = a nha / lúc trước có giải r nên ko giải lại rắc rối

Answer 2

\(N=\frac{1}{1\cdot2}+\frac{1}{2\cdot3}+\cdot\cdot\cdot+\frac{1}{a\left(a+1\right)}\)

\(\Rightarrow N=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\cdot\cdot\cdot+\frac{1}{a}-\frac{1}{a-1}\)

\(\Rightarrow N=1-\frac{1}{a-1}\)

\(\Rightarrow N=\frac{a-1-1}{a-1}\)

\(\Rightarrow N=\frac{a-2}{a-1}\)