OpenOlympiad
Concept 9 of 10Foundation
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Constructing parallel lines

To draw a line parallel to given line ℓ through an external point P:

  1. Draw any transversal from P meeting ℓ at point Q.
  2. Measure the angle at Q (between ℓ and PQ).
  3. At P, construct an EQUAL angle on the same side of PQ.
  4. That new line is parallel to ℓ (by corresponding-angles converse).

With a set square, the process is even faster: slide the set square along a ruler holding one edge fixed along ℓ — the parallel edge gives parallel lines effortlessly.

Example
Railway tracks stay parallel because their spacing is maintained — a real-world use of parallel construction.
💡 Tip:Two lines are proven parallel when corresponding angles (or alternate angles) are equal on a transversal.
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🎯 Try it!

5 questions to check what you just read.

0 / 5
  1. Q1.Two lines are parallel if a transversal creates:
  2. Q2.Quick tool for parallels via sliding:
  3. Q3.Parallel lines are always:
  4. Q4.In a parallelogram, opposite sides are:
  5. Q5.If alternate interior angles are equal, lines are: