1 + 2 cdot 2 + 3 cdot 2^2 + 4 cdot 2^3 + dots | vedclass

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(C) Let $S = 1 + 2 \cdot 2 + 3 \cdot 2^2 + 4 \cdot 2^3 + \dots + 100 \cdot 2^{99} \quad \dots(1)$Multiply by $2$:$2S = 1 \cdot 2^2 + 2 \cdot 2^2 + 3 \

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$1 + 99 \cdot 2^{100}$

Vedclass · Answer

(C) Let $S = 1 + 2 \cdot 2 + 3 \cdot 2^2 + 4 \cdot 2^3 + \dots + 100 \cdot 2^{99} \quad \dots(1)$Multiply by $2$:$2S = 1 \cdot 2^2 + 2 \cdot 2^2 + 3 \cdot 2^3 + \dots + 99 \cdot 2^{99} + 100 \cdot 2^{100} \quad \dots(2)$Subtract $(1)$ from $(2)$:$2S - S = 100 \cdot 2^{100} - (1 + 2 + 2^2 + 2^3 + \dots + 2^{99})$$S = 100 \cdot 2^{100} - \frac{1(2^{100} - 1)}{2 - 1}$$S = 100 \cdot 2^{100} - 2^{100} + 1$$S = 99 \cdot 2^{100} + 1$

$1 + 2 \cdot 2 + 3 \cdot 2^2 + 4 \cdot 2^3 + \dots + 100 \cdot 2^{99} = \dots$

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