If $l, m, n$ are three lines such that $l \parallel m$ and $n \perp l$, prove that $n \perp m$.
Given:
$l, m, n$ are three lines such that $l \parallel m$ and $n \perp l$.
To do:
We have to prove that $n \perp m$.
Solution:

$n \perp l$
This implies,
$\angle 1 = 90^o$
$l \parallel m$ and $n$ is the transversal.
Therefore,
$\angle l = \angle 2$ (Corresponding angles are equal)
$\angle 2 = 90^o$
This implies,
$n \perp m$.
Hence proved.
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