Sign in
Please select an account to continue using cracku.in
↓ →
Join Our JEE Preparation Group
Prep with like-minded aspirants; Get access to free daily tests and study material.
Let $$m_1$$ and $$m_2$$ be the slopes of the tangents drawn from the point $$P(4, 1)$$ to the hyperbola $$H : \frac{y^2}{25} - \frac{x^2}{16} = 1$$. If $$Q$$ is the point from which the tangents drawn to $$H$$ have slopes $$|m_1|$$ and $$|m_2|$$ and they make positive intercepts $$\alpha$$ and $$\beta$$ on the $$x-$$axis, then $$\frac{(PQ)^2}{\alpha\beta}$$ is equal to _____.
Correct Answer: 8
We are given the hyperbola $$H: \frac{y^2}{25} - \frac{x^2}{16} = 1$$ and the point $$P(4, 1)$$.
Here $$a^2 = 25$$ and $$b^2 = 16$$.
For a hyperbola $$\frac{y^2}{a^2} - \frac{x^2}{b^2} = 1$$, the tangent line $$y = mx + c$$ satisfies the condition $$c^2 = a^2 - b^2 m^2$$. Let us verify this.
Substituting $$y = mx + c$$ into $$\frac{y^2}{25} - \frac{x^2}{16} = 1$$:
$$\frac{(mx+c)^2}{25} - \frac{x^2}{16} = 1$$
$$16(mx+c)^2 - 25x^2 = 400$$
$$(16m^2 - 25)x^2 + 32mcx + 16c^2 - 400 = 0$$
For tangency, discriminant = 0: $$(32mc)^2 = 4(16m^2 - 25)(16c^2 - 400)$$
Expanding: $$1024m^2c^2 = 1024m^2c^2 - 25600m^2 - 1600c^2 + 40000$$
$$25600m^2 + 1600c^2 = 40000$$
$$16m^2 + c^2 = 25$$
So the tangent condition is $$c^2 = 25 - 16m^2$$.
The tangent through $$P(4,1)$$: since $$1 = 4m + c$$, we get $$c = 1 - 4m$$.
$$(1-4m)^2 = 25 - 16m^2$$
$$1 - 8m + 16m^2 = 25 - 16m^2$$
$$32m^2 - 8m - 24 = 0$$
$$4m^2 - m - 3 = 0$$
$$(4m + 3)(m - 1) = 0$$
So $$m_1 = 1$$ and $$m_2 = -\frac{3}{4}$$.
Therefore $$|m_1| = 1$$ and $$|m_2| = \frac{3}{4}$$.
Now we find the tangent lines with slopes $$|m_1| = 1$$ and $$|m_2| = \frac{3}{4}$$ that make positive x-intercepts.
For slope 1: $$c^2 = 25 - 16 = 9$$, so $$c = \pm 3$$. The x-intercept is $$-\frac{c}{m} = -c$$. For a positive intercept, we need $$c = -3$$. Tangent: $$y = x - 3$$, with $$\alpha = 3$$.
For slope $$\frac{3}{4}$$: $$c^2 = 25 - 16 \cdot \frac{9}{16} = 25 - 9 = 16$$, so $$c = \pm 4$$. The x-intercept is $$-\frac{c}{3/4} = -\frac{4c}{3}$$. For a positive intercept, we need $$c = -4$$. Tangent: $$y = \frac{3}{4}x - 4$$, with $$\beta = \frac{16}{3}$$.
Point $$Q$$ is the intersection of these two tangent lines:
$$x - 3 = \frac{3}{4}x - 4$$
$$\frac{x}{4} = -1 \Rightarrow x = -4, \quad y = -7$$
So $$Q = (-4, -7)$$.
$$(PQ)^2 = (4-(-4))^2 + (1-(-7))^2 = 64 + 64 = 128$$
$$\alpha\beta = 3 \times \frac{16}{3} = 16$$
$$\frac{(PQ)^2}{\alpha\beta} = \frac{128}{16} = 8$$
The answer is $$8$$.
Predict your JEE Main percentile, rank & performance in seconds
Educational materials for JEE preparation
Ask our AI anything
AI can make mistakes. Please verify important information.
AI can make mistakes. Please verify important information.