Basic proportionality theorem
A line drawn parallel to one side of a triangle divides the other two sides in the same ratio.
Practice This ConceptMain explanation
Teacher explanation
This theorem is a ratio rule inside a triangle. The parallel line creates smaller triangles that share angles with the bigger triangle, so the cut sides stay proportional.
Example
If DE ∥ BC in triangle ABC, then AD/DB = AE/EC.
Simple analogy
Parallel line, proportional cut.
Common confusion
Students often write the side ratio in the wrong order or forget that the line must be parallel.
Exam tip
First check the parallel mark, then write the ratio of the two cut sides carefully.
Answer writing and exam use
1-mark use
Write the exact meaning of basic proportionality theorem in one clean line.
2-mark use
Define basic proportionality theorem and add one example or condition.
3-mark use
Explain basic proportionality theorem, show the method or example, and mention the common mistake.
Practice this concept with focused MCQs
Open the concept quiz intro first, review the test details, and then start a focused MCQ set from this concept only. Instant score and answer review are live now.
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