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# how am i solving this geometric series wrong?

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I want to solve 3.c)

the answer is $$- {209 715 \over 256}$$, what am i doing wrong?

Jul 6, 2018

#1
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how am i solving this geometric series wrong?

$$\begin{array}{|rcll|} \hline && (-4)^{10} \\ &=& {\color{red}+}4^{10} \qquad \text{ because the exponent is even!} \\ &=& 1048576 \\ \hline \end{array}$$

Jul 6, 2018
#2
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C)

Sum = 1/256 x [ 1 - (-4)^10] / [1 - (-4)]

Sum = 1/256 x [1 - 1,048,576] / [1 + 4]

Sum = 1/256 x [-1,048,575] / [5]

Sum = 1/256 x [ - 209,715]

Sum = - 209,715 / 256

Jul 6, 2018