Let’s take an example

Given a line
l
with point P not on it
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With P as center, and any radius,
draw an arc intersecting the line at points A and B
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Now with A as center, and
same radius
draw an arc.
With B as center, and same radius as before, draw an arc
 Mark the point of intersection of the two arcs as point Q
Join P & Q.
∴ PQ is the line perpendicular to line l