A parallelogram ABCD has angle ABC = 118° and angle BAC = 37°. The parallelogram is folded along the diagonal AC so that vertex B lands on the point B', which lies on the same side of AC as D. Find angle DAB'.
Need a hint?
Find angle DAC first using a parallel pair of lines, then compare it with the folded angle B'AC on the same side of AC.
Worked answer
- Angle BCA = 180° − 118° − 37° = 25° (angle sum of triangle ABC).
- Angle DAC = angle BCA = 25° (alternate angles, AD ∥ BC).
- After folding, angle B'AC = angle BAC = 37°, and B' is on the same side of AC as D.
- Both angles are measured from AC on the same side, so angle DAB' = 37° − 25° = 12°.
Answer: 12 °
The mistake this question tests
Habit
Angle facts (straight line, triangle, quadrilateral)
Pupils add the two angles (37° + 25° = 62°) instead of subtracting them, forgetting that B' and D are on the same side of AC after folding.
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