Tangent line and normal on a parabola

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Homework Help Overview

The discussion revolves around a problem involving a parabola defined by the equation y² = 4ax. Participants are tasked with showing a specific relationship between parameters p and q related to points on the parabola and the normals at those points.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants attempt to substitute coordinates into equations but express difficulty in simplifying the results. There are questions about the correctness of the problem setup and the derived expressions. Some participants also explore the implications of the locus of intersection points of tangents.

Discussion Status

The discussion is ongoing, with participants sharing their attempts and questioning the validity of their results. There is no clear consensus, but some guidance has been offered regarding the interpretation of the problem and the relationships between the variables.

Contextual Notes

Some participants express uncertainty about the accuracy of the problem statement and the derived coordinates, indicating potential issues with the original problem setup.

sooyong94
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Homework Statement


If the normal at P(ap^2 ,2ap) to the parabola y^2 = 4ax meets the curve again at Q(aq^2, 2aq), show that p^2 +pq+2=0

Homework Equations


Point-slope form

The Attempt at a Solution


Capture.jpg

I tried putting y=2aq and x=aq^2 but I can seem to simplify the whole thing other than dividing both sides by a
 
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sooyong94 said:

Homework Statement


If the normal at P(ap^2 ,2ap) to the parabola y^2 = 4ax meets the curve again at Q(aq^2, 2aq), show that p^2 +pq+2=0

Homework Equations


Point-slope form

The Attempt at a Solution


Capture.jpg

I tried putting y=2aq and x=aq^2 but I can seem to simplify the whole thing other than dividing both sides by a
The expression is also dividable by q-p.
 
Strangely enough I got this:
Capture.png
 
sooyong94 said:
Strangely enough I got this:
Capture.png
There should be minus in front of the last term in the first equation. You made a mistake when copying.
 
Thanks - worked that out quickly.
 
Now I'm stuck on the second part:

Show that the equation of the locus of the point of intersection of the tangents at P and Q to the parabola is y^2(x+2a)+4a^3 =0. What does this mean? Does this mean that the tangents at P and Q meet at a point?

I managed to find the points of intersection of the two tangents (apq, a(p+q)), but I can't seem to continue at this point.
 
sooyong94 said:
Show that the equation of the locus of the point of intersection of the tangents at P and Q to the parabola is y^2(x+2a)+4a^3 =0. What does this mean? Does this mean that the tangents at P and Q meet at a point?

I managed to find the points of intersection of the two tangents (apq, a(p+q)), but I can't seem to continue at this point.
You have to work with x and y. What are p and q now?
 
x=apq
y=a(p+q)
 
sooyong94 said:
x=apq
y=a(p+q)
Have you copied the question correctly? These x and y values do not fulfill the equation given y^2(x+2a)+4a^3 =0.
 
  • #10
I'm sorry - but I can't catch it.
 
  • #11
sooyong94 said:
I'm sorry - but I can't catch it.
I mean the problem might be wrong. The coordinates of the point of intersection do not fit to the given locus.
 

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