Pre-U
Pre-U 9795/1
2012
June
Q13
6 marks
Challenging +1.8
13 Define the repunit number, \(R _ { n }\), to be the positive integer which consists of a string of \(n 1\) '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 7's.
Pre-U
Pre-U 9795/2
2012
June
Q8
8 marks
Challenging +1.2
8
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An aircraft carrier, \(A\), is heading due north at \(40 \mathrm {~km} \mathrm {~h} ^ { - 1 }\). A destroyer, \(D\), which is 8 km south-west of \(A\), is ordered to take up a position 3 km east of \(A\) as quickly as possible. The speed of \(D\) is \(60 \mathrm {~km} \mathrm {~h} ^ { - 1 }\) (see diagram). Find the bearing, \(\theta\), of the course that \(D\) should take, giving your answer to the nearest degree.