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.
Pre-U
Pre-U 9794/1
2016
June
Q12
10 marks
Standard +0.3
12 A patch of disease on a leaf is being chemically treated. At time \(t\) days after treatment starts, its length is \(x \mathrm {~cm}\) and the rate of decrease of its length is observed to be inversely proportional to the square root of its length. At time \(t = 3 , x = 4\) and, at this instant, the length is decreasing at 0.05 cm per day.
Write down and solve a differential equation to model this situation. Hence find the time it takes for the length to decrease to 0.01 cm .
[0pt]
[10]