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Simplify: sum_(n=1)^(oo)12((2)/(5))^(n-1)

Simplify: n=112(25)n1\sum_{n=1}^{\infty}12\left(\frac{2}{5}\right)^{n-1}

Full solution

Q. Simplify: n=112(25)n1\sum_{n=1}^{\infty}12\left(\frac{2}{5}\right)^{n-1}
  1. Rewrite series in geometric format: Simplify the series formula to a geometric series format.\newlineThe series can be rewritten as 12×n=1(25)n112 \times \sum_{n=1}^{\infty}\left(\frac{2}{5}\right)^{n-1}.\newlineThis is a geometric series with the first term a=(25)0=1a = \left(\frac{2}{5}\right)^0 = 1 and common ratio r=25r = \frac{2}{5}.
  2. Apply sum formula for geometric series: Apply the formula for the sum of an infinite geometric series.\newlineThe sum SS of an infinite geometric series where |r| < 1 is given by S=a1rS = \frac{a}{1 - r}.\newlineHere, a=1a = 1 and r=25r = \frac{2}{5}, so S=1125=135=53S = \frac{1}{1 - \frac{2}{5}} = \frac{1}{\frac{3}{5}} = \frac{5}{3}.
  3. Calculate final sum: Multiply the result by 1212 to find the final sum of the original series.\newlineThe final sum is 12×(53)=2012 \times \left(\frac{5}{3}\right) = 20.

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