# How to express this series in summation notation? 27+18+12+8

Aug 13, 2018

The answer is $= {\sum}_{k = 0}^{3} {2}^{k} \cdot {3}^{3 - k}$
$S = \left({2}^{0} \times {3}^{3}\right) + \left({2}^{1} \times {3}^{2}\right) + \left({2}^{2} \times {3}^{1}\right) + \left({2}^{3} \times {3}^{0}\right)$
$= {\sum}_{k = 0}^{3} {2}^{k} \cdot {3}^{3 - k}$