Question
Find the sum to infinity of $21+14+\frac{28}{3}+\ldots$
✓
Answer
$\begin{aligned} & 21+14+\frac{28}{3}+\ldots \\ & \text { Here } a=21, r=\frac{14}{21}=\frac{2}{3} \\ & S_{\infty}=\frac{a}{1-r} \\ & =\frac{21}{1-\frac{2}{3}} \\ & =\frac{21}{\frac{3-2}{3}} \\ & =\frac{21}{\frac{1}{3}} \\ & =21 \times 3=63 \\ & S_{\infty}=63\end{aligned}$
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