m∠1 + m∠2 + m∠3 = 180°         Sum of angles of a triangle is 180°



m∠4 + m∠3 = 180°               Supplementary angles add to 180°



m∠1 + m∠2 - m∠4 = 0           Subtracted bottom eqn from top eqn



m∠1 + m∠2 = m∠4               Added m∠4 to both sides, (DONE) 

 

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