1. Problem statement: Find the equation of the normal to the curve $\ln(2x^2 - 7)$ at the point where the curve crosses the positive x-axis.
2. Find the x-intercept by setting the curve equal to 0 because the x-axis is $y=0$.
$$\ln(2x^2 - 7) = 0$$
Exponentiate both sides to remove the logarithm.
$$2x^2 - 7 = e^0 = 1$$
Solve for $x$ by isolating $x^2$.
$$2x^2 = 8$$
Show the division step and simplify the fraction using cancellation.
$$x^2 = \frac{8}{2}$$
$$x^2 = \frac{\cancel{8}}{\cancel{2}} = 4$$
Take square roots to find the x-intercepts.
$$x = \pm 2$$
The positive x-intercept is $x = 2$, so the point of interest is $$(2,0)$$.
3. Compute the derivative to get the slope of the tangent using the chain rule; for $y=\ln(u)$ we have $y'=u'/u$.
$$y' = \frac{d}{dx}\ln(2x^2 - 7) = \frac{4x}{2x^2 - 7}$$
4. Evaluate the derivative at $x=2$ to obtain the tangent slope.
$$y'(2) = \frac{4\cdot 2}{2\cdot 2^2 - 7} = \frac{8}{8-7} = \frac{8}{1}$$
Show the simplification using cancellation.
$$\frac{8}{1} = \frac{\cancel{8}}{\cancel{1}} = 8$$
So the tangent slope at $(2,0)$ is $8$.
5. The normal line has slope equal to the negative reciprocal of the tangent slope.
$$m_{n} = -\frac{1}{8}$$
6. Use the point-slope form of a line for the normal through the point $$(2,0)$$.
$$y - 0 = -\frac{1}{8}(x - 2)$$
Clear denominators by multiplying both sides by 8.
$$8y = -(x - 2)$$
Expand the right-hand side.
$$8y = -x + 2$$
Rearrange to integer-coefficient form $ax + by + c = 0$.
$$x + 8y - 2 = 0$$
7. Final answer: $x + 8y - 2 = 0$.
Normal Equation Ee93F1
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