Finding the Line Normal to a Hyperbola: Domenic's Q&A at Yahoo Answers

In summary, we are given a hyperbola with an equation of x^2/4 - y^2/9 = 1. We are asked to find the coordinates of a point on the curve with x=4 and y>0, which we determine to be (4,3sqrt(3)). Then, we find the slope of the normal at this point to be -1/sqrt(3) and use the point-slope formula to find the equation of the normal in general form, which is x+sqrt(3)y-13=0.
  • #1
MarkFL
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Here is the question:

An Hyperbola has the equation x^2/4 - y^2/9 = 1?


- Find the coordinates of the point on this curve with x=4, y>0
- Find the slope of the normal to the curve at this point, and hence find the equation of the normal. Give the equation in general form, ie. Ax+By+C=0

I have posted a link there to this thread so the OP can see my work.
 
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  • #2
Hello Domenic,

We are given the hyperbola:

\(\displaystyle \frac{x^2}{4}-\frac{y^2}{9}=1\)

Let's multiply through by $36$ to obtain:

(1) \(\displaystyle 9x^2-4y^2=36\)

Letting $x=4$, we find:

\(\displaystyle 9(4)^2-4y^2=36\)

Divide through by 4:

\(\displaystyle 36-y^2=9\)

\(\displaystyle y^2=27\)

Since we are told $y>0$, taking the positive root, we find:

\(\displaystyle y=3\sqrt{3}\)

Hence, the coordinates of the point are:

\(\displaystyle \left(4,3\sqrt{3} \right)\)

Now, to find the normal slope, we need to implicitly differentiate (1) with respect to $-y$ to get:

\(\displaystyle 18x\left(-\frac{dx}{dy} \right)-8y(-1)=0\)

\(\displaystyle -\frac{dx}{dy}=-\frac{4y}{9x}\)

Thus, the slope of the normal line at the given point is:

\(\displaystyle \left. -\frac{dx}{dy} \right|_{(x,y)=\left(4,3\sqrt{3} \right)}=-\frac{4\left(3\sqrt{3} \right)}{9(4)}=-\frac{1}{\sqrt{3}}\)

Now, we have a point on the normal line and the slope, thus the point-slope formula yields:

\(\displaystyle y-3\sqrt{3}=-\frac{1}{\sqrt{3}}(x-4)\)

Multiply through by $-\sqrt{3}$:

\(\displaystyle -\sqrt{3}y+9=x-4\)

Arrange in the required standard form:

\(\displaystyle x+\sqrt{3}y-13=0\)

Here is a plot of the hyperbola and the normal line at the given point:

View attachment 1658
 

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FAQ: Finding the Line Normal to a Hyperbola: Domenic's Q&A at Yahoo Answers

What is a hyperbola?

A hyperbola is a type of conic section, similar to a parabola or ellipse, that is defined by the equation x^2/a^2 - y^2/b^2 = 1. It has two branches that are symmetrical about the x-axis.

How do you find the line normal to a hyperbola?

To find the line normal to a hyperbola at a specific point, you can use the formula y = mx + b, where m is the slope of the tangent line at that point and b is the y-intercept. The normal line will be perpendicular to the tangent line, so the slope of the normal line will be the negative reciprocal of the tangent line's slope.

What is the difference between the line normal and the tangent line on a hyperbola?

The tangent line is a line that touches the hyperbola at a specific point and has the same slope as the hyperbola at that point. The normal line, on the other hand, is perpendicular to the tangent line and intersects the hyperbola at a 90 degree angle. It is used to find the slope of the tangent line at a given point on the hyperbola.

Can the line normal to a hyperbola be a vertical line?

Yes, it is possible for the line normal to a hyperbola to be a vertical line. This would occur when the hyperbola is oriented in such a way that its tangent line at a specific point is horizontal. In this case, the normal line would be perpendicular to the horizontal tangent line and therefore would be a vertical line.

How is finding the line normal to a hyperbola useful?

Finding the line normal to a hyperbola is useful in many applications, such as engineering and physics. It can be used to determine the direction of a force acting on an object at a given point on a hyperbola, or to calculate the acceleration of an object moving along a hyperbola. It is also used in the study of optics and the reflection of light rays off of curved surfaces.

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