Let \(g(x)=\sqrt{12x-x^2}\).
a) Find the largest real domain \(D_g\) and the equation of the normal line \(n\) to the graph of \(g\) at \(x_1=2\).
b) Show algebraically that every normal line to the graph of \(g\), including those at the endpoints, passes through \(M=(6,0)\).
c) Find the point \(P\) on the graph of \(g\), with \(x<6\), where the tangent line is parallel to \(y=x\).
Hints
- A normal slope is the negative reciprocal of the tangent slope when both are finite and nonzero.
- Write a general normal equation at \(x_0\), and test whether \(M\) satisfies it.
- Treat horizontal, vertical, and endpoint tangents as separate cases.
- A tangent parallel to \(y=x\) has slope \(1\).
- Check solutions in the original radical equation after squaring.
Solution
1. a) Require \(x(12-x)\ge0\), so \(D_g=[0,12]\). For \(0<x<12\), \(g'(x)=\frac{6-x}{\sqrt{12x-x^2}}\).
2. At \(x=2\), \(g(2)=2\sqrt{5}\), and the tangent slope is \(\frac{2}{\sqrt{5}}\). The normal slope is \(-\frac{\sqrt{5}}{2}\), so \(n:y=-\frac{\sqrt{5}}{2}x+3\sqrt{5}\).
3. b) For \(0<x_0<12\) and \(x_0\ne6\), the normal slope is \(m_n=\frac{\sqrt{12x_0-x_0^2}}{x_0-6}\). Substituting \(M=(6,0)\) into the normal equation gives \(0=m_n(6-x_0)+g(x_0)\), which is true.
4. At \(x_0=6\), the tangent is horizontal, so the normal is \(x=6\), which contains \(M\). At the endpoints \((0,0)\) and \((12,0)\), the tangents are vertical, so the normal is the x-axis, which also contains \(M\).
5. c) Parallelism to \(y=x\) requires \(g'(x)=1\). Thus, \(6-x=\sqrt{12x-x^2}\). Squaring gives \(x^2-12x+18=0\), with candidates \(x=6\pm3\sqrt{2}\).
6. The original unsquared equation requires \(x<6\), so \(x=6-3\sqrt{2}\). Then \(g(x)=3\sqrt{2}\), giving \(P=(6-3\sqrt{2},3\sqrt{2})\).
Answer
a) \(D_g=[0,12]\); \(n:y=-\frac{\sqrt{5}}{2}x+3\sqrt{5}\)
b) Every normal contains \(M=(6,0)\); the interior, center, and endpoint cases all satisfy the claim.
c) \(P=(6-3\sqrt{2},3\sqrt{2})\)