OCR MEI C2 2015 June — Question 11 12 marks

Exam BoardOCR MEI
ModuleC2 (Core Mathematics 2)
Year2015
SessionJune
Marks12
PaperDownload PDF ↗
Mark schemeDownload PDF ↗
TopicGeometric Sequences and Series
TypeCompound growth applications
DifficultyStandard +0.3 This is a straightforward geometric sequence/series application with clear structure: (i) requires simple GP term calculation (3^8), (ii) uses standard GP sum formula, (iii) involves logarithms but the inequality is given to prove (not derive), and (iv) repeats the calculation with different common ratio. All parts follow standard textbook methods with no novel insight required, making it slightly easier than average.
Spec1.04i Geometric sequences: nth term and finite series sum1.04j Sum to infinity: convergent geometric series |r|<11.06f Laws of logarithms: addition, subtraction, power rules

11 Jill has 3 daughters and no sons. They are generation 1 of Jill's descendants.
Each of her daughters has 3 daughters and no sons. Jill's 9 granddaughters are generation 2 of her descendants. Each of her granddaughters has 3 daughters and no sons; they are descendant generation 3. Jill decides to investigate what would happen if this pattern continues, with each descendant having 3 daughters and no sons.
  1. How many of Jill's descendants would there be in generation 8 ?
  2. How many of Jill's descendants would there be altogether in the first 15 generations?
  3. After \(n\) generations, Jill would have over a million descendants altogether. Show that \(n\) satisfies the inequality $$n > \frac { \log _ { 10 } 2000003 } { \log _ { 10 } 3 } - 1 .$$ Hence find the least possible value of \(n\).
  4. How many fewer descendants would Jill have altogether in 15 generations if instead of having 3 daughters, she and each subsequent descendant has 2 daughters? \section*{END OF QUESTION PAPER}

11 Jill has 3 daughters and no sons. They are generation 1 of Jill's descendants.\\
Each of her daughters has 3 daughters and no sons. Jill's 9 granddaughters are generation 2 of her descendants. Each of her granddaughters has 3 daughters and no sons; they are descendant generation 3.

Jill decides to investigate what would happen if this pattern continues, with each descendant having 3 daughters and no sons.\\
(i) How many of Jill's descendants would there be in generation 8 ?\\
(ii) How many of Jill's descendants would there be altogether in the first 15 generations?\\
(iii) After $n$ generations, Jill would have over a million descendants altogether. Show that $n$ satisfies the inequality

$$n > \frac { \log _ { 10 } 2000003 } { \log _ { 10 } 3 } - 1 .$$

Hence find the least possible value of $n$.\\
(iv) How many fewer descendants would Jill have altogether in 15 generations if instead of having 3 daughters, she and each subsequent descendant has 2 daughters?

\section*{END OF QUESTION PAPER}

\hfill \mbox{\textit{OCR MEI C2 2015 Q11 [12]}}