Question

Find the equation of a straight line passing through $(1,2)$ and perpendicular to the line $y=7$

Answer 1

The given line $y=7$ .... (1)
The required line $\perp$ on to $\left(1\right)$
The required line is
$x=k$
It passes through $(1,2)$
$1=k$
$k=1$
The required line is $x=1$.