For Sigma-summation Underscript n = 1 Overscript infinity EndScripts StartFraction 0.2 n Over 0.8 EndFraction, find S3=

How to do this , and can some one explain

Identify the initial amount a and the growth factor b in the exponential function. a(x)=680*4.3^x

36x2 + 49y2 = 1,764 the foci are located at: (-√13, 0) and (√13,0) (0, -√13) and (0,√13) (-1, 0) and (1, 0)edit: the answer is (- the square root of 13, 0) and (the square root of 13, 0)