# How do I use summation notation to write the series 2.2 + 8.8?

Sep 20, 2014

This very short series appears to be geometric, with a ratio of 4, and a first term of 2.2.

2.2 + 8.8 = 11

I would write: S_2=((2.2(1-4^2))/(1-4)))
from the explicit formula ${S}_{n} = \left(\frac{{a}_{1} \left(1 - {r}^{n}\right)}{1 - r}\right)$. In the formula, ${a}_{1}$ represents the first term, r stands for the ratio between terms, and ${S}_{n}$ means "the sum of n terms".

You can also use Sigma, or summation notation: where i=1 indicates to start with substituting in i = 1 as the exponent of 4, and multiplying by 2.2. The 2 at the top of the Sigma indicates that one should stop summing when one substitutes in 1 = 2, which would be the second term to add.