Find an Equation of the Tangent Lines Passing Through a Poin

[logan3]

Homework Statement

Find an equation of the tangent lines [in point-slope form] to the hyperbola x^2 + y^2 = 16 that pass through the point (2, -2).

Homework Equations

Point-slope form: y - y_1 = m(x - x_1)
Slope: m = \frac {y_1 - y_0} {x_1 - x_0}

The Attempt at a Solution

First, take the derivative using implicit differentiation.

D_x: 2x + 2yy' = 0 \Rightarrow y' = \frac {x} {y}

Since the derivative is the general form of the slope of a line at any of its points, then we can put it equal to the equation for slope. Furthermore, the derivative is tangent to a particular point on the original equation, which we can call point (x_1, y_1). Since we found the equation of the slope at this point to be y' = \frac {x} {y}, then plugging in the point (x_1, y_1) (a particular case) should also work: y' = \frac {x_1} {y_1}. Finally, we were given our second point above as (2, -2).

y' = \frac {x_1} {y_1} = \frac {y_1 + 2} {x_1 - 2} \Rightarrow x_1^2 - 2x_1 = y_1^2 + 2y_1 \Rightarrow 2x_1 - 2y_1 = x_1^2 + y_1^2

Since the general equation x^2 + y^2 = 16 works for any pair of points on the line, then it should also work for the particular case of the point on the tangent line that is intersecting it, (x_1, y_1).

2x_1 - 2y_1 = x_1^2 + y_1^2 = 16 \Rightarrow x_1 - y_1 = 8 \Rightarrow y_1 = x_1 - 8

Substitute into x_1^2 + y_1^2 = 16 and solve for x_1 and y_1. We find x_1 = 5, y_1 = 3 and m = \frac 5 3. Hence, the equation of the tangent line is y - 3 = \frac 5 3 (x - 5).

It took me awhile to reason through this problem, so I wasn't sure if it was correct. Thank-you.

 

[Mark44]

Homework Statement

Find an equation of the tangent lines [in point-slope form] to the hyperbola x^2 + y^2 = 16

What you wrote is a circle. Did you mean $x^2 - y^2 = 16$?

that pass through the point (2, -2).

Homework Equations

Point-slope form: y - y_1 = m(x - x_1)
Slope: m = \frac {y_1 - y_0} {x_1 - x_0}

The Attempt at a Solution

First, take the derivative using implicit differentiation.

D_x: 2x + 2yy' = 0 \Rightarrow y' = \frac {x} {y}

The above isn't consistent with the equation you showed (which itself could be incorrect). If you differentiate $x^2 + y^2 = 16$ w.r.t. x, you get 2x + 2yy' = 0, or y' = -x/y.

Since the derivative is the general form of the slope of a line at any of its points, then we can put it equal to the equation for slope. Furthermore, the derivative is tangent to a particular point on the original equation, which we can call point (x_1, y_1). Since we found the equation of the slope at this point to be y' = \frac {x} {y}, then plugging in the point (x_1, y_1) (a particular case) should also work: y' = \frac {x_1} {y_1}. Finally, we were given our second point above as (2, -2).

y' = \frac {x_1} {y_1} = \frac {y_1 + 2} {x_1 - 2} \Rightarrow x_1^2 - 2x_1 = y_1^2 + 2y_1 \Rightarrow 2x_1 - 2y_1 = x_1^2 + y_1^2

Since the general equation x^2 + y^2 = 16 works for any pair of points on the line, then it should also work for the particular case of the point on the tangent line that is intersecting it, (x_1, y_1).

2x_1 - 2y_1 = x_1^2 + y_1^2 = 16 \Rightarrow x_1 - y_1 = 8 \Rightarrow y_1 = x_1 - 8

Substitute into x_1^2 + y_1^2 = 16 and solve for x_1 and y_1. We find x_1 = 5, y_1 = 3 and m = \frac 5 3. Hence, the equation of the tangent line is y - 3 = \frac 5 3 (x - 5).

It took me awhile to reason through this problem, so I wasn't sure if it was correct. Thank-you.