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3.4 Sequences and Series (A-level only)

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Question 32

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

3.4 Sequences and Series (A-level only) Questions

  1. A Level
  2. /Maths
  3. /3.4 Sequences and Series (A-level only)

231 exam-style questions on WJEC A Level Maths 3.4 Sequences and Series (A-level only), covering 3.4.1 Sequences and Series (A-level only), 3.4.2 Sequences and Series (A-level only), 3.4.3 Sequences and Series (A-level only), 3.4.4 Sequences and Series (A-level only), 3.4.5 Sequences and Series (A-level only), 3.4.6 Sequences and Series (A-level only), and 3.4.7 Sequences and Series (A-level only). Each one has a worked solution and a mark scheme showing where the marks go.

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