User interface language: English | Español

Date May 2017 Marks available 6 Reference code 17M.2.SL.TZ2.S_5
Level Standard Level Paper Paper 2 Time zone Time zone 2
Command term Find Question number S_5 Adapted from N/A

Question

Consider a geometric sequence where the first term is 768 and the second term is 576.

Find the least value of n such that the n th term of the sequence is less than 7.

Markscheme

* This question is from an exam for a previous syllabus, and may contain minor differences in marking or structure.

attempt to find r     (M1)

eg 576 768 ,   768 576 ,   0.75

correct expression for u n     (A1)

eg 768 ( 0.75 ) n 1

EITHER (solving inequality)

valid approach (accept equation)     (M1)

eg u n < 7

valid approach to find n     M1

eg 768 ( 0.75 ) n 1 = 7 ,   n 1 > log 0.75 ( 7 768 ) , sketch

correct value

eg n = 17.3301     (A1)

n = 18 (must be an integer)     A1     N2

OR (table of values)

valid approach     (M1)

eg u n > 7 , one correct crossover value

both crossover values, u 17 = 7.69735 and u 18 = 5.77301     A2

n = 18 (must be an integer)     A1     N2

OR (sketch of functions)

valid approach     M1

eg sketch of appropriate functions

valid approach     (M1) 

eg finding intersections or roots (depending on function sketched)

correct value

eg n = 17.3301     (A1)

n = 18 (must be an integer)     A1     N2

[6 marks]

Examiners report

[N/A]

Syllabus sections

Topic 1—Number and algebra » SL 1.3—Geometric sequences and series
Show 72 related questions
Topic 1—Number and algebra

View options