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1.7.5 Geometric sequences and series (A-level only)

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Question 39

A specialized industrial centrifuge extracts a mineral from a solution such that the mass extracted each hour forms a geometric series. The theoretical maximum mass that can be extracted over an infinite time period is 150 grams. In the first hour, the centrifuge extracts aaa grams, where a>80a > 80a>80. In the second hour, it extracts 36 grams.

a.

Determine the value of the first term, aaa, and the common ratio, rrr, of the series.

[5]
bi.

Show that the mass extracted in the nnnth hour, MnM_nMn​, can be written as

Mn=2n⋅325n−2 M_n = \frac{2^n \cdot 3^2}{5^{n-2}} Mn​=5n−22n⋅32​
[3]
bii.

Hence show that

log⁡2Mn=n(1−log⁡25)+(2log⁡23+2log⁡25) \log_2 M_n = n(1 - \log_2 5) + (2 \log_2 3 + 2 \log_2 5) log2​Mn​=n(1−log2​5)+(2log2​3+2log2​5)
[3]

1.7.5 Geometric sequences and series (A-level only) Questions

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