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1.7 D: Sequences and series

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Question 28

A chemical reaction is monitored at regular time intervals, and the concentration of the remaining catalyst, CCC, is found to follow a geometric progression with first term aaa and common ratio rrr, where r>0r > 0r>0.

Given that:

  • the concentration at the 2nd interval is 54 mg/L
  • the concentration at the 4th interval is 24 mg/L
a.

Show that r=23r = \frac{2}{3}r=32​.

[2]
b.

Determine the value of aaa.

[1]
c.

Given that the sum of the concentrations recorded over the first nnn intervals is greater than 242.5 mg/L

Find the smallest possible value of nnn.

(Solutions based entirely on graphical or numerical methods are not acceptable.)

[4]
Markscheme

1.7 D: Sequences and series Questions

  1. A Level
  2. /Maths
  3. /1.7 D: Sequences and series

305 exam-style questions on AQA A Level Maths 1.7 D: Sequences and series, covering 1.7.1 Binomial expansion, 1.7.2 Types of sequence (A-level only), 1.7.3 Sigma notation (A-level only), 1.7.4 Arithmetic sequences and series (A-level only), 1.7.5 Geometric sequences and series (A-level only), and 1.7.6 Sequences and series in modelling (A-level only). Each one has a worked solution and a mark scheme showing where the marks go.

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