# Math Help - Use sigma notation to write the sum

1. ## Use sigma notation to write the sum

(x^4/8) - (x^5/9) + (x^6/10) - (x^7/11) + (x^8/12)

2. Usualy when you want to write a sum that way you look for a rule...
$\sum_{i=4}^{8}(-1)^i\cdot x^\frac{i}{i+4}$ or $\sum_{i=1}^{5}(-1)^{i+1}\cdot x^\frac{i+3}{i+7}$.

3. $\sum_{i=4}^{8}(-1)^i\cdot \frac{x^i}{i+4}$ or $\sum_{i=1}^{5}(-1)^{i+1}\cdot \frac{x^{i+3}}{i+7}$.