Angles in a triangle when nothing is given
Triangles
Finding three unknown angles in a triangle using algebra and the angle sum, when no angle is given directly.
Challenge level
Exam standard
01
Have a go first
The Question
In triangle ABC, angle A is twice angle B, and angle C is 20° more than angle B. Find all three angles.
02
Then check together
The Answer
Reveal the Answer
A = 80°, B = 40°, C = 60°
Let B = x. Then A = 2x and C = x + 20. Sum: 4x + 20 = 180, so x = 40. A bar model works here too — three bars summing to 180.
Now watch the solution
Pencil and paper
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