JEE Advanced · Mathematics · 13. Parabola

Paragraph:

Let $P Q$ be a focal chord of the parabola $y^{2}=4 a x$. The tangents to the parabola at $P$ and $Q$ meet at a point lying on the line $y=2 x+a, a>0$.

Question:

If chord $P Q$ subtends an angle $\theta$ at the vertex of $y^{2}=4 a x$, then $\tan \theta=$

A $\frac{2}{3} \sqrt{7}$
B $\frac{- 2}{3} \sqrt{7}$
C $\frac{2}{3} \sqrt{5}$
D $\frac{- 2}{3} \sqrt{5}$

Verified Solution

Answer & Solution

Correct Answer

(D) $\frac{- 2}{3} \sqrt{5}$

Step-by-step Solution

Detailed explanation

t - \frac{1}{t} = - 1

\Rightarrow t + \frac{1}{t} = \sqrt{5}

tan θ = \frac{\frac{2}{t} + 2 t}{1 - 4} = \frac{2 (\frac{1}{t} + t)}{- 3} = \frac{2 \sqrt{5}}{- 3}

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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	List I		List II
A.	a =	P.	13
B.	b =	Q.	-3
C.	c =	R.	1
D.	d =	S.	-2

List - I	List - II
(A)	(P)
(B)	(Q)
(C)	(R)
(D)	(S)
	(T)

Paragraph:

Let \(P Q\) be a focal chord of the parabola \(y^{2}=4 a x\). The tangents to the parabola at \(P\) and \(Q\) meet at a point lying on the line \(y=2 x+a, a>0\).

Question:

If chord \(P Q\) subtends an angle \(\theta\) at the vertex of \(y^{2}=4 a x\), then \(\tan \theta=\)

Answer & Solution

Step-by-step Solution

See the Complete Solution

Paragraph: Let \(P Q\) be a focal chord of the parabola \(y^{2}=4 a x\). The tangents to the parabola at \(P\) and \(Q\) meet at a point lying on the line \(y=2 x+a, a>0\). Question: If chord \(P Q\) subtends an angle \(\theta\) at the vertex of \(y^{2}=4 a x\), then \(\tan \theta=\)

Step-by-step Solution

Paragraph:

Let \(P Q\) be a focal chord of the parabola \(y^{2}=4 a x\). The tangents to the parabola at \(P\) and \(Q\) meet at a point lying on the line \(y=2 x+a, a>0\).

Question:

If chord \(P Q\) subtends an angle \(\theta\) at the vertex of \(y^{2}=4 a x\), then \(\tan \theta=\)