- #1
issacnewton
- 1,000
- 29
Homework Statement
Given an ellipse ##\frac{x^2}{a^2} + \frac{y^2}{b^2} = 1## , where ##a \ne b##, find the equation of the set of all points from which there are two tangents to the curve whose slopes are (a) reciprocals and (b) negative reciprocals
Homework Equations
Equation of a tangent to an ellipse at a point ##(x_o, y_o)## is ##\frac{x x_o}{a^2} + \frac{y y_o}{b^2}=1##
The Attempt at a Solution
Here is my attempt for the solution for (a). Let ##(h,k)## be the point from where we will have the two tangents to the ellipse ##\frac{x^2}{a^2} + \frac{y^2}{b^2} = 1##. Let these two tangents meet the ellipse at points ##(x_1,y_1)## and ##(x_2, y_2)##. So the equations of these tangents are ##\frac{x x_1}{a^2} + \frac{y y_1}{b^2}=1## and ##\frac{x x_2}{a^2} + \frac{y y_2}{b^2}=1##. Now ##(h,k)## lies on these tangents. So we must have ##\frac{h x_1}{a^2} + \frac{k y_1}{b^2}=1## and ##\frac{h x_2}{a^2} + \frac{k y_2}{b^2}=1##. Also the slopes of these tangents are ##m_1 = -\left( \frac{b^2 x_1}{a^2 y_1} \right)## and ##m_2 = -\left( \frac{b^2 x_2}{a^2 y_2} \right)##. Since the product of the slopes is 1, we must have $$\left( \frac{b^2 x_1}{a^2 y_1} \right)\left( \frac{b^2 x_2}{a^2 y_2} \right) = 1\cdots\cdots(1)$$ Also the points ##(x_1, y_1)## and ##(x_2, y_2)## lie on the ellipse, so they satisfy the equation of the ellipse $$\frac{x_1^2}{a^2} + \frac{y_1^2}{b^2} = 1\cdots\cdots(2)$$
$$\frac{x_2^2}{a^2} + \frac{y_2^2}{b^2} = 1\cdots\cdots(3)$$ After this I did lot of algebra and tried to get an equation involving only ##h,k,a,b## , but I am unable to eliminate the variables ##x_1,y_1,x_2,y_2##. I think at this point I have all the information I need to solve this problem. Any hints to further this problem will be helpful.
Thanks