Draw two intersecting lines passing through the point P and which is perpendicular to l.
To prove: Only one perpendicular line can be drawn through a given point i.e., to prove ∠P=0∘.
In ΔAPB, ∠A+∠P+∠B=180∘
[by angle sum property of a triangle]
So, lines n and m coincide.
Hence, only one perpendicular line can be drawn through a give point.