Higher November 2020 Paper 1R Q18
18 In an arithmetic series, the 6th term is 39
In the same arithmetic series, the 19th term is 7.8
Work out the sum of the first 25 terms of the arithmetic series.
(4)
| Scheme | Marks |
|---|---|
| \(a + 5d = 39\) or \(a + 18d = 7.8\) or \(13d = -31.2\) oe | M1 |
| \(a = 51\) or \(d = -2.4\) | A1 |
e.g. \(\dfrac{25}{2}(2 \times 51 + (25 - 1) \times -2.4)\) oe or \(12.5(2a + 23d + d) = 12.5(39 + 7.8 - 2.4)\) oe | M1 |
| 555 | A1 |
| (4) | |
| (4 marks) |
Notes
M1: for substituting their values for \(a\) and \(d\) into \(S_n\), \(a\) and \(d\) must be clearly stated.