Find the equation of the line which is perpendicular to the line 3x + 2y = 8 and passes through the midpoint of the line joining the points (6, 4) and (4, - 2).
Given: The given line is $3 x+2 y=8$. The perpendicular line passes through the midpoint of $(6,4)$ and $(4,-2)$.
Formulae to be used: The product of slopes of two perpendicular lines $=-1$.
If (a,b) and (c,d) be two points, then their midpoint is given by
$\left(\frac{a+c}{2}, \frac{b+d}{2}\right)$
The slope of this line is $-3 / 2$.
∴ the slope of the perpendicular line =
$\frac{-1}{-3 / 2}=2 / 3$
The equation of the line can be written in the form
$y=\left(\frac{2}{3}\right) x+c$
(c is the $y$ - intercept)
This line passes through the midpoint of $(6,4)$ and $(4,-2)$.
The co - ordinates of the midpoint of the line joining the given points is
$\left(\frac{6+4}{2}, \frac{4+(-2)}{2}\right)=(5,1)$
(5,1) satisfies the equation
$y=\left(\frac{2}{3}\right) x+c$
$\therefore 1=\left(\frac{2}{3}\right) \times 5+\mathrm{c}$ or, $\mathrm{c}=1-\frac{10}{3}=-\frac{7}{3}$
The required equation is
$y=\left(\frac{2}{3}\right) x+\left(-\frac{7}{3}\right)$
i.e. $2 x-3 y=7$