Tacy drew a triangle to explain the relationship between the angles of a triangle and an exterior angle. She labeled the interior angles 1, 2, and 3. She labeled as 4 the exterior angle that is adjacent to angle 2. Triangle with angles labeled 1, 2, 3, & exterior angle at angle 2 is labeled 4.

What should Stacy do to explain the relationship between an exterior angle and the interior angles in a triangle?

Stacy should first show that m∠1+m∠3=m∠2+m∠4. Then she should subtract to show that m∠1=m∠2 and m∠3=m∠4.

Stacy should first show that m∠4=m∠2. Then, since m∠1+m∠2+m∠3=180∘, she should substitute to show m∠1+m∠4+m∠3=180∘.

Stacy should first show that m∠4+m∠2=m∠1+m∠2+m∠3. Then she should subtract to show that m∠4=m∠1+m∠3.

Stacy should first show that m∠4=m∠3. Then, since m∠1+m∠2+m∠3=180∘, she should substitute to show m∠1+m∠2+m∠4=180∘.