Question:hard

If the normal to the rectangular hyperbola \[ x^2-y^2=1 \] at the point \(P\left(\frac{\pi}{4}\right)\) meets the curve again at \(Q(\theta)\), then \[ \sec^2\theta+\tan\theta= \]

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For the rectangular hyperbola \[ x^2-y^2=1, \] the parametric point is \[ (\sec\theta,\tan\theta) \] and the slope of normal can be found using implicit differentiation.
Updated On: Jun 22, 2026
  • \(43\)
  • \(57\)
  • \(3\)
  • \(1\)
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The Correct Option is B

Solution and Explanation

Step 1: Locate the point $P$.
For $x^2-y^2=1$ a point is $(\sec\phi,\tan\phi)$. At $\phi=\frac{\pi}{4}$ we get $P = (\sec\frac{\pi}{4},\tan\frac{\pi}{4}) = (\sqrt2,\,1)$.
Step 2: Find the slope of the normal at $P$.
Differentiating $x^2-y^2=1$ gives $\frac{dy}{dx} = \frac{x}{y}$, so at $(\sqrt2,1)$ the tangent slope is $\sqrt2$. The normal slope is therefore $-\frac{1}{\sqrt2}$.
Step 3: Write the normal line.
The normal through $(\sqrt2,1)$ is $y - 1 = -\frac{1}{\sqrt2}(x - \sqrt2)$, which simplifies to $x + \sqrt2\,y = 2\sqrt2$.
Step 4: Use the point $Q(\theta)=(\sec\theta,\tan\theta)$.
Since $Q$ lies on the normal, $\sec\theta + \sqrt2\tan\theta = 2\sqrt2$. The point $Q$ already satisfies the curve. We need $\sec^2\theta + \tan\theta$.
Step 5: Solve for $\tan\theta$.
From $\sec\theta = 2\sqrt2 - \sqrt2\tan\theta$ and $\sec^2\theta - \tan^2\theta = 1$: $(2\sqrt2-\sqrt2\tan\theta)^2 - \tan^2\theta = 1$, i.e. $8 - 8\tan\theta + 2\tan^2\theta - \tan^2\theta = 1$, so $\tan^2\theta - 8\tan\theta + 7 = 0$. This gives $\tan\theta = 7$ (taking the new point, since $\tan\theta=1$ is $P$). Then $\sec^2\theta = 1 + \tan^2\theta = 50$.
Step 6: Combine the required quantity.
Therefore $\sec^2\theta + \tan\theta = 50 + 7 = 57$.
\[ \boxed{57} \]
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