Foundation November 2019 Paper 2 Q27
27 Here is an identity.
\(a(3x - 10) \equiv 21x + 2b\)
Work out the values of \(a\) and \(b\). [3 marks]
| Answer | Mark | Comments |
|---|---|---|
| \(a = 7\) | B2 | B1 \(3ax - 10a\) or \(3ax = 21x\) or \(3ax - 21x = 0\) or \(3a = 21\) or \(3a - 21 = 0\) or \(21 \div 3\) oe or \(-10a = 2b\) oe |
| \(b = -35\) | B1ft | ft \(-5 \times\) their \(a\) where \(a \ne 0\) |
Additional guidance
| Ignore collection error if correct expansion seen eg \(3ax - 10a - 21x + 2b = 0\) (should be \(-\,2b\)) | B1 |
| Ignore incorrect simplification if correct expansion seen eg \(3ax - 10a = -7ax\) | B1 |
| Allow eg \(a \times 3x\) for \(3ax\) | |
| Allow eg \(a3x\) for \(3ax\) | |
| Embedded 7 with \(a = 7\) not stated eg \(7(3x - 10)\) or \(7 \times 3x = 21x\) or \(21 \div 7 = 3\) | B1 |
| Allow B1 even if not subsequently used |