What are the equations for finding common tangents between two curves?

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SUMMARY

The discussion focuses on deriving the equations for common tangents between the curves defined by \(y^{2}=4ax\) and \(x^{2}=4by\). Participants emphasize the importance of equating the derivatives of both curves, leading to the equation \(\frac{2a}{y}=\frac{x}{2b}\). The solution approach involves using the two-point form of the straight line equation, but participants encounter complexities due to cubic roots. A recommended strategy is to first derive the general tangent equations for each curve before finding their common tangents.

PREREQUISITES
  • Understanding of calculus, specifically derivatives of functions.
  • Familiarity with the equations of conic sections, particularly parabolas.
  • Knowledge of the two-point form of a straight line equation.
  • Basic algebraic manipulation skills, including handling cubic roots.
NEXT STEPS
  • Derive the general equation for a tangent line to the curve \(y^{2}=4ax\).
  • Derive the general equation for a tangent line to the curve \(x^{2}=4by\).
  • Explore methods for solving cubic equations to simplify the tangent derivation process.
  • Research geometric interpretations of common tangents between parabolas.
USEFUL FOR

Students studying calculus, mathematicians interested in conic sections, and educators teaching tangent lines and their applications in geometry.

labinojha
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Homework Statement



Finding the equations of common tangents of y^{2}=4ax and x^{2}=4by

Homework Equations



Equating both curves' derivatives :
\frac{2a}{y}=\frac{x}{2b}

The Attempt at a Solution



Calculated both of the curves' derivatives and equated them .
Used the resulting relation in two point form of straight line equation (y-y1)=m(x-x1) .

Got Stuck now! with too many cuberoots .
 
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In your equation: \displaystyle \frac{2a}{y}=\frac{x}{2b}\,, the y is from a point on the first curve, the x from a point on the second curve.

Instead, start by doing the following:

Find the general equation for a line tangent to y2=42x.

The find the general equation for a line tangent to x2=4by.
 
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