Find the distance of the line 4x + 7y + 5 = 0 from the point (1, 2) along the line 2x – y = 0.
The given lines are
2x – y = 0 … (1)
4x + 7y + 5 = 0 … (2)
A (1, 2) is a point on line (1).
Let B be the point of intersection of lines (1) and (2).
On solving equations (1) and (2), we obtain $x=\frac{-5}{18}$ and $y=\frac{-5}{9}$.
$\therefore$ Coordinates of point B are $\left(\frac{-5}{18}, \frac{-5}{9}\right)$.
By using distance formula, the distance between points A and B can be obtained as
$\mathrm{AB}=\sqrt{\left(1+\frac{5}{18}\right)^{2}+\left(2+\frac{5}{9}\right)^{2}}$ units
$=\sqrt{\left(\frac{23}{18}\right)^{2}+\left(\frac{23}{9}\right)^{2}}$ units
$=\sqrt{\left(\frac{23}{2 \times 9}\right)^{2}+\left(\frac{23}{9}\right)^{2}}$ units
$=\sqrt{\left(\frac{23}{9}\right)^{2}\left(\frac{1}{2}\right)^{2}+\left(\frac{23}{9}\right)^{2}}$ units
$=\sqrt{\left(\frac{23}{9}\right)^{2}\left(\frac{1}{4}+1\right) \text { units }}$
$=\sqrt{\left(\frac{23}{9}\right)^{2}\left(\frac{1}{4}+1\right)}$ units
$=\frac{23}{9} \sqrt{\frac{5}{4}}$ units
$=\frac{23}{9} \times \frac{\sqrt{5}}{2}$ units
$=\frac{23 \sqrt{5}}{18}$ units
Thus, the required distance is $\frac{23 \sqrt{5}}{18}$ units