Finding tangent lines that pass through given points

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Homework Help Overview

The problem involves finding the coordinates of points on the curve defined by f(x) = x³, such that the tangent lines at these points pass through a specified point (a, 0). The discussion centers around the mathematical properties of tangent lines and their relationship to the curve.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the need to find the derivative of the function to establish the slope of the tangent lines. There is uncertainty about how to formulate the equations for the tangent lines and whether multiple tangent lines can pass through the same external point. Some participants express confusion about the graphical implications of the problem.

Discussion Status

Participants are actively engaging with the problem, exploring different approaches to formulate the equations of the tangent lines. Some have suggested that there may be two tangent lines that can pass through the point (a, 0), while others are working through the algebraic setup to find specific coordinates.

Contextual Notes

There is mention of potential constraints regarding the number of tangent lines and the graphical interpretation of the curve, as well as the specific point through which the tangents must pass. The discussion reflects a mix of attempts and clarifications regarding the problem setup.

methionine
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Homework Statement


Find the co-ordinates of all points on the curve f(x)= x3 whose tangent lines pass through the point (a,0)


Homework Equations


f '(x) = nxn-1


The Attempt at a Solution


I am really not sure how to attack this question. My initial thoughts are to find f '(x) then, given the points (a,0), create a point-slope equation for a line. But, this would only give me one equation. On the other hand, when I look at the graph of x^3 I can't see how two tangent lines could share a point. I'm not sure how I would find the co-ordinates either.

f '(x) = 3x2

y = 3x2 (x-a)
 
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methionine said:

Homework Statement


Find the co-ordinates of all points on the curve f(x)= x3 whose tangent lines pass through the point (a,0)


Homework Equations


f '(x) = nxn-1


The Attempt at a Solution


I am really not sure how to attack this question. My initial thoughts are to find f '(x) then, given the points (a,0), create a point-slope equation for a line. But, this would only give me one equation. On the other hand, when I look at the graph of x^3 I can't see how two tangent lines could share a point. I'm not sure how I would find the co-ordinates either.

f '(x) = 3x2

y = 3x2 (x-a)
... and y = x3 .

Hello methionine. Welcome to PF !
 
I'm sorry, what do you mean? "...and y = x3
 
methionine said:
I'm sorry, what do you mean? "...and y = x3
Your line passes through the point (x, x3) as well as the point (a, 0).
 
methionine said:
On the other hand, when I look at the graph of x^3 I can't see how two tangent lines could share a point.

I believe there are indeed two lines tangent to the curve that go through the point (a,0). Look carefully.
 
Hey guys, I spent the last day mulling this question over in my head, and decided to attack it in a slightly different way.

Basically, did what I was doing up to this point, set a point-slope form equation using x and x and x3 and x1 and y1...

y-x3 = 3x2(x-x3)

Now, I know the line has to pass through (a,0) as well, so I plugged those values into X and Y and ended up with two points.

-x3 = 3x2(a-x3)

Kind of an interesting question for me. I hope my work checks out!
 

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