2026 · Basic · Set 2 · Part 3·Q34·5 marks

Triangles

Question

State and prove "Basic Proportionality Theorem."

Approach

Statement: If a line is drawn parallel to one side of a triangle intersecting the other two sides, then it divides the two sides in the same ratio. Proof involves constructing perpendiculars and using area ratios of triangles with same height to establish proportionality.

Step-by-step working

  1. 1

    State Basic Proportionality Theorem

    1 mark
  2. 2

    Draw figure and construction

    1 mark
  3. 3

    Use area ratios of triangles

    2 marks
  4. 4

    Establish

    1 mark

Concepts used

thales theoremproportionalityparallel linesproof

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