Sum to infinity of an infinite GP (Infinite sum exists only if |r|<1; then S = a/(1−r))

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Infinite sum exists only if |r|<1; then S = a/(1−r)

What each symbol means

SymbolWhat it stands for
aFirst term
rCommon ratio

When to use this

Convergent only for |r|<1; for |r|≥1 the series does not converge (except trivial a=0).