Shortcut Trick
Identify the angle relationships based on parallel lines properties:
• ∠1 = ∠7 (Alternate Exterior Angles) → Always True.
• ∠8 = ∠6 (Vertically Opposite Angles) → Always True.
• ∠1 = ∠5 (Corresponding Angles) → Always True.
• ∠2 and ∠5 are supplementary (∠2 + ∠5 = 180°) because ∠2 = ∠6 (corresponding) and ∠5 + ∠6 = 180° (linear pair).
Thus, ∠2 = ∠5 is only true if the transversal is perpendicular.
∴ The correct answer is ∠2 = ∠5.
Alternate Method
Given: Line l || m and a transversal line intersecting them.
Formula Used: When two parallel lines are cut by a transversal:
- Corresponding angles are equal.
- Alternate interior/exterior angles are equal.
- Vertically opposite angles are equal.
- Linear pair angles are supplementary (sum to 180°).
Calculations:
⇒ From the figure, ∠1 and ∠5 are corresponding angles. Since l || m, ∠1 = ∠5.
⇒ ∠1 and ∠7 are alternate exterior angles. Since l || m, ∠1 = ∠7.
⇒ ∠8 and ∠6 are vertically opposite angles. Vertically opposite angles are always equal.
⇒ Now, consider ∠2 and ∠5. ∠2 and ∠6 are corresponding angles, so ∠2 = ∠6.
⇒ ∠5 and ∠6 form a linear pair on line m, so ∠5 + ∠6 = 180°.
⇒ Substituting ∠6 with ∠2, we get ∠5 + ∠2 = 180°.
⇒ Since ∠5 and ∠2 are supplementary, they are only equal if both are 90°.
⇒ Therefore, the relation ∠2 = ∠5 is not necessarily true for any general transversal.
∴ The correct answer is ∠2 = ∠5.
Additional Information
Consecutive Interior Angles
Angles on the same side of the transversal inside the parallel lines (like ∠4 and ∠5) are supplementary: ∠4 + ∠5 = 180°.
Alternate Interior Angles
Pairs of angles on opposite sides of the transversal between the two parallel lines (like ∠3 and ∠5) are always equal: ∠3 = ∠5.
Transversal Properties
If the transversal is perpendicular to the parallel lines, all eight angles will be right angles (90°).