HOTS Mathematics Primary 5 Angles

In the figure, triangle ABC has AB = AC. Point D is on side BC such that AD = BD. Given that ∠ABD = 35°, find ∠DAC.

A 110°
B 35°
C 75°
D 70°
Show Worked Solution

Worked Solution

Step 1: Triangle ABD is isosceles (AD = BD), so its base angles are equal: angle BAD = angle ABD = 35°. Step 2: Triangle ABC is also isosceles (AB = AC), so its base angles are equal: angle ABC = angle ACB. Since D is on BC, angle ABC = angle ABD = 35°. Step 3: Apex angle BAC = 180° − 35° − 35° = 110°. Step 4: Angle DAC = angle BAC − angle BAD = 110° − 35° = 75°. Answer: 75°

Correct answer: 75°

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