Let's fill in the blanks in your proof step by step:
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Given the diagram shown, \( \angle 1 \cong \angle 5 \); \( \angle 2 \cong \angle 4 \) because alternate interior angles are congruent when lines are parallel.
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Then, I know that \( m\angle 1 = m\angle 5 \); \( m\angle 2 = m\angle 4 \) because congruent angles have equal measures.
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\( m\angle 1 + m\angle 3 + m\angle 4 = 180^\circ \) by the definition of a straight angle.
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Finally, \( m\angle 1 + m\angle 3 + m\angle 2 = 180^\circ \) by substitution.
Thus, the final filled proof is:
Given the diagram shown, \( \angle 1 \cong \angle 5 \); \( \angle 2 \cong \angle 4 \) because alternate interior angles are congruent when lines are parallel. Then, I know that \( m\angle 1 = m\angle 5 \); \( m\angle 2 = m\angle 4 \) because congruent angles have equal measures. \( m\angle 1 + m\angle 3 + m\angle 4 = 180^\circ \) by the definition of a straight angle. Finally, \( m\angle 1 + m\angle 3 + m\angle 2 = 180^\circ \) by substitution.
This completes the proof that \( m\angle 1 + m\angle 3 + m\angle 2 = 180^\circ \).