June 2025 Paper 1 Q6
6 An arithmetic series has first term 3 and common difference \(-0.5\)
Find the sum of the first 250 terms. [2 marks]
| Scheme | Marks | AO |
|---|---|---|
| Substitutes \(n = 250\) in \(\dfrac{n}{2}\) and \((n-1)\) and at least one of \(a = 3\) or \(d = -0.5\) into \(\dfrac{n}{2}\big(2a + (n-1)d\big)\) Or into \(a + (n-1)d\) and \(\dfrac{n}{2}(a + l)\) note the correct value of \(l = -121.5\) Or into \(\displaystyle\sum_{1}^{250} a + (n-1)d\) Condone poor bracketing | M1 | 1.1a |
| Obtains \(-14812.5\) or \(-\dfrac{29625}{2}\) | A1 | 1.1b |
| (2 marks) |