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From module Triangle Angle Sum
Given ∠1+∠2=180∘\angle 1 + \angle 2 = 180^{\circ}∠1+∠2=180∘ and ∠3=∠4\angle 3 = \angle 4∠3=∠4. Find ∠4\angle 4∠4. [See figure in original PDF]
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