Pre-U
Pre-U 9795/1
2016
Specimen
Q13
6 marks
Challenging +1.8
13 Define the repunit number, \(R _ { n }\), to be the positive integer which consists of a string of \(n 1 \mathrm {~s}\). Thus,
$$R _ { 1 } = 1 , \quad R _ { 2 } = 11 , \quad R _ { 3 } = 111 , \quad \ldots , \quad R _ { 7 } = 1111111 , \quad \ldots , \text { etc. }$$
Use induction to prove that, for all integers \(n \geqslant 5\), the number
$$13579 \times R _ { n }$$
contains a string of ( \(n - 4\) ) consecutive 7s.