Specimen 2020 Paper 2 Q15
15 Here is a sketch of the curve \(\quad y = ab^{-x} \quad\) where \(a\) and \(b\) are positive constants.
(0, 3) and (2, 0.48) lie on the curve.
Not drawn accurately
Work out the values of \(a\) and \(b\). [4 marks]
| Answer | Mark | Comments |
|---|---|---|
| \(a = 3\) | B1 | |
| \(0.48 =\) their \(3 \times b^{-2}\) | M1 | oe |
| \(b^2 = \dfrac{\text{their } 3}{0.48}\) or \(b^2 = 6.25\) \(\sqrt{\dfrac{\text{their } 3}{0.48}}\) or \(\sqrt{6.25}\) | M1dep | oe |
| \(b = 2.5\) | A1ft | ft B0M2 |