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The equation of a tangent to the parabola $$y^2 = 8x$$ is $$y = x + 2$$. The point on this line from which the other tangent to the parabola is perpendicular to the given tangent is
The standard form of the given parabola is $$y^{2}=4ax$$.
Comparing $$y^{2}=8x$$ with $$y^{2}=4ax$$ gives $$a=2$$.
For $$y^{2}=4ax$$ the tangent in slope form is
$$y = mx + \frac{a}{m} \qquad -(1)$$
1. Tangent already given
The equation $$y = x + 2$$ has slope $$m_1 = 1$$.
With $$m = 1$$, (1) gives $$c = \dfrac{a}{m}= \dfrac{2}{1}=2$$, so $$y = x + 2$$ is indeed a tangent.
2. Slope of the required second tangent
The second tangent must be perpendicular to the first.
If $$m_1 = 1$$, then $$m_2 = -\dfrac{1}{m_1} = -1$$.
3. Equation of the perpendicular tangent
Put $$m = -1$$ in (1):
$$y = (-1)x + \frac{2}{-1} \; \Rightarrow \; y = -x - 2 \qquad -(2)$$
4. Point of intersection of the two tangents
Solve $$y = x + 2$$ and $$y = -x - 2$$ simultaneously:
$$x + 2 = -x - 2 \;\Longrightarrow\; 2x = -4 \;\Longrightarrow\; x = -2$$
With $$x = -2$$, $$y = x + 2 = 0$$.
Thus the point common to both tangents (and lying on the given tangent) is $$(-2,0)$$.
Hence, the required point is $$(-2,0)$$.
Option D which is: $$(-2, 0)$$
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