Tính nhanh \[A = \frac{5} + \frac{5} + \frac{5} + ... + \frac{5}\]

\[\begin{array}{*{20}{l}}{A = \frac{5} + \frac{5} + \frac{5} + ... + \frac{5}}\\{ = 5.\left( {\frac{1} + \frac{1} + \frac{1} + ... + \frac{1}} \right)}\end{array}\]

\[ = \frac{5}{2}.\left( {1 - \frac{1}{3} + \frac{1}{3} - \frac{1}{5} + \frac{1}{5} - \frac{1}{7} + ... + \frac{1} - \frac{1}} \right)\]

\[\begin{array}{*{20}{l}}{ = \frac{5}{2}.\left( {1 - \frac{1}} \right)}\\{ = \frac{5}{2}.\frac = \frac.}\end{array}\]

Đáp án cần chọn là: D