Question

# The general equation to a system of parallel chords in the parabola ${{y}^{2}}=\dfrac{25}{7}x$ is $4x-y+k=0$. What is the equation to the corresponding diameter?

Hint: The diameter of a parabola is defined as a line bisecting the system of parallel chords of a parabola. For ${{y}^{2}}=4ax$, it is given by $y=\dfrac{2a}{m}$ where $m$ is the slope of the chords.

It is given in the question that the general equation to a system of parallel chords in the parabola ${{y}^{2}}=\dfrac{25}{7}x$ is $4x-y+k=0$.
We know that the general equation of a straight line is given by $y=mx+c$, where the term $m$represents the slope of the line and the term $c$ represents the intercept.
We can convert the equation $4x-y+k=0$ to the general form by rearranging the terms as below,
$y=4x+k$
When we compare the two equations, we get that the slope as $m=4$.
We know that the general equation of the parabola is given by ${{y}^{2}}=4ax$. Now, let us consider the equation of the parabola given in the question, ${{y}^{2}}=\dfrac{25}{7}x$.
Now, let us convert the given equation into the general form. For that, we have to multiply and divide the RHS by $4$. This can be done as shown below,
\begin{align} & {{y}^{2}}=\dfrac{4}{4}\times \dfrac{25}{7}x \\ & {{y}^{2}}=4\times \left( \dfrac{25}{4\times 7} \right)x \\ & {{y}^{2}}=4\times \left( \dfrac{25}{28} \right)x \\ \end{align}
On comparing the above equation of the parabola with the general form, we get that $a=\dfrac{25}{28}$.
We know that the equation of diameter of the parabola is given by $y=\dfrac{2a}{m}$. Therefore, we can substitute the values of $m=4$ and $a=\dfrac{25}{28}$ in it to get the diameter.
So, we get the diameter as
\begin{align} & y=\dfrac{2\times \dfrac{25}{28}}{4} \\ & y=\dfrac{\dfrac{25}{14}}{4} \\ & y=\dfrac{25}{56} \\ \end{align}
Therefore, the diameter of the parabola obtained is $y=\dfrac{25}{56}$.
Note: Diameter of a parabola is also defined as the locus of all the mid-points of a system of parallel chords of a parabola. So, there is an alternate method to solve this question. From the equation of chord, $x$ can be substituted in the equation of parabola to formulate a quadratic equation. The roots of which will be the ordinates of point of intersection. The locus of midpoint of these ordinates would give the diameter.