A circle with centre C has equation \((x - 8)^2 + (y - 4)^2 = 100\).
(a) Show that the point \(T(-6, 6)\) is outside the circle.
Two tangents from \(T\) to the circle are drawn.
(b) Show that the angle between one of the tangents and \(CT\) is exactly \(45^\circ\).
The two tangents touch the circle at \(A\) and \(B\).
(c) Find the equation of the line \(AB\), giving your answer in the form \(y = mx + c\).
(d) Find the \(x\)-coordinates of \(A\) and \(B\).
Solution
(a) Calculate the distance from \(T(-6, 6)\) to the center \(C(8, 4)\):
\((x - 8)^2 + (y - 4)^2 = (-6 - 8)^2 + (6 - 4)^2 = 200\)
\(\sqrt{200} = 10\), hence \(\sqrt{200} > 10\), so \(T\) is outside the circle.
(b) Use the formula for the angle between a tangent and a radius:
\(\text{angle} = \sin^{-1}\left(\frac{10}{10\sqrt{2}}\right) = 45^\circ\)
(c) Find the gradient \(m\) of \(CT\):
\(m = \frac{1}{7}\)
Equation of \(AB\) is \(y - 5 = 7(x - 1)\)
\(y = 7x - 2\)
(d) Substitute \(y = 7x - 2\) into the circle's equation:
\((x - 8)^2 + (7x - 2 - 4)^2 = 100\)
\(50x^2 - 100x = 0\)
\(x = 0\) and \(x = 2\)
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