June 2022 Paper 1 Q5
5 \(\quad (3x + a)(5x - 4) \equiv 15x^2 - 2x + b\)
Work out the values of \(a\) and \(b\). [3 marks]
| Answer | Mark | Comments |
|---|---|---|
| \(15x^2 - 12x + 5ax - 4a\) or \(5ax - 12x = -2x\) or \(5a - 12 = -2\) or \(b = -4a\) | M1 | oe |
| (\(a =\)) 2 | A1 | |
| (\(b =\)) \(-4 \times\) their \(a\) | A1ft | \(-8\), but do not award \(-8\) unless it comes from \(a = 2\) |
Additional guidance
| Candidates who use substitutions for \(x\) are likely to use \(x = 0\) and gain M1. Award M1 for any number substituted in correctly to gain an equation in \(a\) and \(b\) |