CAIE IGCSE Additional Math 8.3 Solve Tangent Problems Questions

gradient of radius $=\frac{-3+1}{4-3}=-2$ soi by gradient
of tangent
gradient of tangent $=\frac{-1}{\text { their }(-2)}$
FT their -2
Allow if differentiation is used e.g.:

$y+1=\frac{1}{2}(x-3)$ oe
$$\text { FT } \frac{-1}{\text { their }-2}$$

ISW from a correct unsimplified answer

Alternative

use of differentiation e.g.:
$$\begin{aligned} (y+3)^{2}=5-(x-4)^{2} y=\sqrt{-x^{2}+8 x-11}-3 \frac{\mathrm{~d} y}{\mathrm{~d} x}=\frac{-2 x+8}{2 \sqrt{-x^{2}+8 x-11}} \end{aligned}$$
allow one error
or use of implicit differentiation (not on syllabus) e.g.:
$2 x+2 y \frac{\mathrm{~d} y}{\mathrm{~d} x}-8+6 \frac{\mathrm{~d} y}{\mathrm{~d} x}=0$
leading to $\frac{\mathrm{d} y}{\mathrm{~d} x}=\frac{8-2 x}{2 y+6}$

Substitute (3,-1) in their $\frac{\mathrm{d} y}{\mathrm{~d} x}$
to get gradient $=\frac{1}{2}$
Dep on first M1

$y+1=\frac{1}{2}(x-3)$ oe
ISW from a correct unsimplified answer