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Triangles
EXERCISE 6.3
CBSE_NCERT_TEXTBOOK
Grade 10

Question:

E is a point on the side AD produced of a parallelogram ABCD and BE intersects CD at F. Show that  ABE ~  CFB.

Step-by-Step Solution

Key Concept: Use the parallelism in a parallelogram (\(AB \parallel CD\) and \(AD \parallel BC\)) to establish two pairs of equal angles, leading to the AA (Angle‑Angle) similarity criterion for triangles.
1. Identify the parallel sides of the parallelogram\:
\[AB \parallel CD \quad\text{and}\quad AD \parallel BC.\]

2. Locate the points\:
- \(E\) lies on the extension of \(AD\) beyond \(D\).
- \(BE\) meets \(CD\) at \(F\).

3. Show that \(\angle ABE = \angle CFB\)\:
- \(\angle ABE\) is the angle formed by the line \(AB\) and the transversal \(BE\).
- Since \(AB \parallel CD\) and \(CF\) lies on \(CD\), the angle between \(AB\) and \(BE\) equals the angle between \(CF\) and \(FB\) (alternate interior angles).\
Hence, \(\angle ABE = \angle CFB\).

4. Show that \(\angle AEB = \angle CBF\)\:
- \(AE\) is a continuation of \(AD\); therefore \(AE\) is collinear with \(AD\).
- Because \(AD \parallel BC\), the angle made by \(AE\) (or \(AD\)) with \(BE\) equals the angle made by \(BC\) with \(BF\) (alternate interior angles).\
Hence, \(\angle AEB = \angle CBF\).

5. Apply the AA similarity criterion\:
- From steps 3 and 4 we have two pairs of equal angles:
\[\angle ABE = \angle CFB \quad\text{and}\quad \angle AEB = \angle CBF.\]
- Therefore, by the AA (Angle‑Angle) criterion, the triangles \(\Delta ABE\) and \(\Delta CFB\) are similar.

6. Conclusion\:
\[\boxed{\Delta ABE \sim \Delta CFB}.\]

Remark: The similarity also implies the proportionality of corresponding sides:
\[\frac{AB}{CF}=\frac{BE}{FB}=\frac{AE}{CB}.\]

Correct Answer: ΔABE ∼ ΔCFB
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