The equation of a curve is \(y = 5 - \frac{8}{x}\).
(i) Show that the equation of the normal to the curve at the point \(P(2, 1)\) is \(2y + x = 4\).
This normal meets the curve again at the point \(Q\).
(ii) Find the coordinates of \(Q\).
(iii) Find the length of \(PQ\).
Solution
(i) Differentiate \(y = 5 - \frac{8}{x}\) to find the gradient of the tangent: \(\frac{dy}{dx} = \frac{8}{x^2}\).
At \(x = 2\), \(\frac{dy}{dx} = \frac{8}{4} = 2\). The gradient of the normal is \(-\frac{1}{2}\) (since \(m_1 m_2 = -1\)).
The equation of the normal is \(y - 1 = -\frac{1}{2}(x - 2)\), which simplifies to \(2y + x = 4\).
(ii) Solve the simultaneous equations \(2y + x = 4\) and \(y = 5 - \frac{8}{x}\).
Substitute \(y = \frac{4 - x}{2}\) into \(y = 5 - \frac{8}{x}\) to get \(\frac{4 - x}{2} = 5 - \frac{8}{x}\).
Multiply through by \(2x\) to clear fractions: \((4 - x)x = 10x - 16\).
Simplify to \(x^2 + 6x - 16 = 0\). Solving gives \(x = -8\) or \(x = 2\). Since \(x = 2\) is point \(P\), \(Q\) is \((-8, 6)\).
(iii) Use the distance formula to find \(PQ\): \(\sqrt{(2 - (-8))^2 + (1 - 6)^2} = \sqrt{10^2 + 5^2} = \sqrt{125}\).
The length of \(PQ\) is approximately \(11.2\).
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