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Nov 2013 p12 q7
850
An athlete runs the first mile of a marathon in 5 minutes. His speed reduces in such a way that each mile takes 12 seconds longer than the preceding mile.
(i) Given that the nth mile takes 9 minutes, find the value of n.
(ii) Assuming that the length of the marathon is 26 miles, find the total time, in hours and minutes, to complete the marathon.
Solution
(i) The time taken for each mile forms an arithmetic sequence where the first term is 5 minutes (or 300 seconds) and the common difference is 12 seconds. The nth term of an arithmetic sequence is given by:
\(a_n = a + (n-1) imes d\)
where \(a = 300\) and \(d = 12\). We need to find \(n\) such that \(a_n = 540\) (since 9 minutes is 540 seconds):
\(540 = 300 + (n-1) imes 12\)
\(240 = (n-1) imes 12\)
\(n-1 = 20\)
\(n = 21\)
(ii) The total time for 26 miles is the sum of the arithmetic sequence with 26 terms. The sum \(S_n\) of an arithmetic sequence is given by: