JEE Advanced · Mathematics · 13. Parabola
Let \(A\) and \(B\) be two distinct points on the parabola \(y^2=4 x\). If the axis of the parabola touches a circle of radius \(r\) having \(A B\) as its diameter, then the slope of the line joining \(A\) and \(B\) can be
- A
\(-\frac{1}{r}\)
- B
\(\frac{1}{r}\)
- C
\(\frac{2}{r}\)
- D
\(-\frac{2}{r}\)
Answer & Solution
Correct Answer
(D)
\(-\frac{2}{r}\)
Step-by-step Solution
Detailed explanation
Here, coordinate \(M=\left(\frac{t_1^2+t_2^2}{2}, t_1+t_2\right) i e\), mid point of chord \(A B\).

\(M P=t_1+t_2=r\) Also, \(\quad m_{A B}=\frac{2 t_2-2 t_1}{t_2^2-t_1^2}=\frac{2^{\ldots(i)}}{t_2+t_1}\) (when \(A B\) is chord)
\[
\Rightarrow \quad m_{A B}=\frac{2}{r}
\]
[from Eq. (i)]
Also, \(\quad m_{A^{\prime} B^{\prime}}=-\frac{2}{r}\) (when \(A^{\prime} B^{\prime}\) is chord)
Hence, (c, d) is the correct option.

\(M P=t_1+t_2=r\) Also, \(\quad m_{A B}=\frac{2 t_2-2 t_1}{t_2^2-t_1^2}=\frac{2^{\ldots(i)}}{t_2+t_1}\) (when \(A B\) is chord)
\[
\Rightarrow \quad m_{A B}=\frac{2}{r}
\]
[from Eq. (i)]
Also, \(\quad m_{A^{\prime} B^{\prime}}=-\frac{2}{r}\) (when \(A^{\prime} B^{\prime}\) is chord)
Hence, (c, d) is the correct option.
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