In figure, AP and BQ are perpendiculars to the line-segment AB and AP = BQ. Prove that O is the midpoint of line segments AB and PQ.

In ∆OAP and ∆OBQ,
AP = BQ | Given
∠OAP = ∠OBQ | Each = 90°
∠AOP = ∠BOQ
| Vertically Opposite Angles
∴ ∆OAP ≅ ∆OBQ | AAS Axiom
∴ OA = OB | C.P.C.T.
and OP = OQ | C.P.C.T.
⇒ O is the mid-point of line segments AB and PQ