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Consider the infinite series $\sum_{n=1}^{\infty }\cfrac{1}{n\left ( n+1 \right )}$. Write out the first four terms of this series.

$1+2+3+4$

$\cfrac{1}{2}+\cfrac{1}{6}+\cfrac{1}{12}+\cfrac{1}{20}$

$\cfrac{1}{2}+\cfrac{1}{5}+\cfrac{1}{10}+\cfrac{1}{17}$

$\cfrac{1}{2}+\cfrac{2}{3}+\cfrac{3}{4}+\cfrac{4}{5}$