Question

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

Answer 1

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=((a_1(1-r^n))/(1-r))`. 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.