Question

Express the series using sigma notation. 10 + 50 + 250 + 1250.

Answer 1

Each next term is being multiplied by 5. Assuming there are only 4 terms to the sum, you have

$10+50+250+1250=10\times5^0+10\times5^1+10\times5^2+10\times5^3=\displaystyle\sum_{i=0}^310\times5^i$