The sketch shows circle C C\,C with centre P(6.5,7)P(6.5, 7)P(6.5,7). The line l l\,l with equation y=12x\displaystyle y = \frac{1}{2}xy=21x is a tangent to C C\,C at point Q(8,4)Q(8, 4)Q(8,4).

Find an equation for CCC.
The line with equation y=12x+k\displaystyle y = \frac{1}{2}x + ky=21x+k, where k k\,k is a non-zero constant, is also a tangent to CCC. Find the value of kkk.
76 exam-style questions on WJEC A Level Maths 1.3 Coordinate geometry in the (x, y) plane, covering 1.3.1 Coordinate geometry in the (x, y) plane, 1.3.2 Coordinate geometry in the (x, y) plane, 1.3.3 Coordinate geometry in the (x, y) plane, and 1.3.4 Coordinate geometry in the (x, y) plane. Each one has a worked solution and a mark scheme showing where the marks go.