Question:

If a normal chord at a point $t$ on the parabola $y^{2}=4 \,a \,x$ subtends a right angle at the vertex, then $t$ equals to

Updated On: Jun 1, 2023
  • 1
  • $\sqrt{2}$
  • 2
  • $\sqrt{3}$
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The Correct Option is B

Solution and Explanation

The perpendicular of the normal to the parabola $y^{2}=4 a x$ at $P$ is
Suppose, it meets the parabola at $Q$. If $O$ be the vertex of the parabola, then the combined equation of $O P$ and $O Q$ is a homogeneous equation of second degree. $y^{2}=4 a x\left(\frac{y+t x}{2 a t+a t^{3}}\right)$ $\Rightarrow y^{2}\left(2 a t+a t^{3}\right)=4 a x(y+t x)$ $\Rightarrow 4 a t x^{2}+4 a x y-\left(2 a t+a t^{3}\right) y^{2}=0$ Since, $O P$ and $O Q$ are at right angles, then Coefficient of $x^{2}+$ Coefficient of $y^{2}=0$ $\therefore 4 a t-2 a t-a t^{3}=0$ $\Rightarrow t^{2}=2$ $ \Rightarrow t=\sqrt{2}$
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Concepts Used:

Straight lines

A straight line is a line having the shortest distance between two points. 

A straight line can be represented as an equation in various forms,  as show in the image below:

 

The following are the many forms of the equation of the line that are presented in straight line-

1. Slope – Point Form

Assume P0(x0, y0) is a fixed point on a non-vertical line L with m as its slope. If P (x, y) is an arbitrary point on L, then the point (x, y) lies on the line with slope m through the fixed point (x0, y0) if and only if its coordinates fulfil the equation below.

y – y0 = m (x – x0)

2. Two – Point Form

Let's look at the line. L crosses between two places. P1(x1, y1) and P2(x2, y2)  are general points on L, while P (x, y) is a general point on L. As a result, the three points P1, P2, and P are collinear, and it becomes

The slope of P2P = The slope of P1P2 , i.e.

\(\frac{y-y_1}{x-x_1} = \frac{y_2-y_1}{x_2-x_1}\)

Hence, the equation becomes:

y - y1 =\( \frac{y_2-y_1}{x_2-x_1} (x-x1)\)

3. Slope-Intercept Form

Assume that a line L with slope m intersects the y-axis at a distance c from the origin, and that the distance c is referred to as the line L's y-intercept. As a result, the coordinates of the spot on the y-axis where the line intersects are (0, c). As a result, the slope of the line L is m, and it passes through a fixed point (0, c). The equation of the line L thus obtained from the slope – point form is given by

y – c =m( x - 0 )

As a result, the point (x, y) on the line with slope m and y-intercept c lies on the line, if and only if

y = m x +c