S∞_\infty∞ = a1−r\;\frac{a}{1-r}1−ra since 1 > r
S∞_\infty∞ = 100, r = −12\;\frac{-1}{2}2−1, a = ?
100 = a1− −12\;\frac{a}{1-\;\frac{-1}{2}}1−2−1a = a32\;\frac{a}{\frac{3}{2}}23a ( - - = +)
a = 100 x 32\;\frac{3}{2}23 = 150
Therefore, the first term (a) = 150
