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The locus of point from where tangents drawn to \( y ^{2}=4 x \) include an angle of \( 45^{\circ} \) is(A) \( x^{2}-y^{2}+6 x-1=0 \)(B) \( x^{2}+y^{2}+6 x+1=0 \)(C) \( x^{2}-y^{2}+6 x+1=0 \)(D) \( x^{2}+y^{2}-6 x+1=0 \) |
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Answer» Option (C) is correct. Given parabola is y2 = 4x ....(1) Let the point from where tangents are drawn be (h, k). i.e., (h, k) is point of intersection of two tangents to parabola y2 = 4x. Tangents of parabola (1) is T : y = mx + \(\frac{1}{m} .....(2)\) (h, k) lies on T. ∴ k = mh + \(\frac{1}{m}\) \(\Rightarrow m^2h - mh + 1 = 0 .....(3)\) which is a quadratic equation in m. Let m1 and m2 are two roots of quadratic equation (3) ∴ \(m_1 + m_2 = \frac{-(-k)}{h} = \frac{k}{h}\) and \(m_1m_2 = \frac{1}{h}.\) ∴ \((m_1 - m_2)^2\) \(= (m_1 + m_2)^2 - 4m_1m_2\) \(= \left(\frac{k}{h}\right)^2 - \frac{4}{h}\) \(= \frac{k^2 - 4h}{h^2}\) ∴ \(m_1 - m_2 = \frac{\sqrt{k^2 - 4h}}{h}.\) Let angle between two tangent be α. ∴ tan α \(= \pm \frac{m_1 - m_2}{1 + m_1m_2}\) \(= \pm \cfrac{\frac{\sqrt{k^2 - 4h}}{h}}{1 + \cfrac{1}{h}}\) \(\Rightarrow\) tan α \(= \pm \frac{\sqrt{k^2 - 4h}}{h + 1}\) \(\Rightarrow\) k2 - 4h = tan2α (h + 1)2 .....(4) But given that tangents include an angle of \(45^\circ.\) i.e., α = \(45^\circ\) \(\Rightarrow\) tan α = tan \(45^\circ\) = 1 ∴ k2 - 4h = (h + 1)2 (From (4)) ∴ Locus of point (h, k) is y2 - 4x = (x + 1)2 \(\Rightarrow\) y2 - 4x = x2 + 2x + 1 \(\Rightarrow\) x2 - y2 + 6x + 1 = 0. |
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