Write the sum using sigma notation:
$\displaystyle {\frac{{{1}}}{{{1}\cdot{2}}}}+{\frac{{{1}}}{{{2}\cdot{3}}}}+{\frac{{{1}}}{{{3}\cdot{4}}}}+\cdots+{\frac{{{1}}}{{{83}\cdot{84}}}}={\sum_{{{n}={1}}}^{{{A}}}}{B}$, where
$\displaystyle {A}=$ Preview Question 6 Part 1 of 2   ,
$\displaystyle {B}=$ Preview Question 6 Part 2 of 2   .