A geometric tile design on a HDB block lobby wall is shaped like the parallelogram ABCD. The angle at vertex A, ∠DAB, measures 65°. What is the size of ∠ABC, the angle at vertex B?
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Worked Solution
Step 1: Recall the property. In a parallelogram, consecutive angles (angles that share a side) are supplementary — they add to 180°.
Step 2: ∠DAB and ∠ABC share the side AB, so they are consecutive angles. ∠DAB + ∠ABC = 180°.
Step 3: Substitute and solve. 65° + ∠ABC = 180°, so ∠ABC = 180° − 65° = 115°.
Check (reverse): 115° + 65° = 180° ✓.
Final answer: ∠ABC = 115°.
Answer: 115°
Correct answer: 115°
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