Explanation: To find the exterior angle x we first need to find the last corner angle (a) of the triangle. The angles of triangles add up to 180° so we can use the formula: 25+55+a=180
Solving formula: 25+55+a=180 80+a=180 (Subtract 80 from both sides) a=100°
Now that we know a=100 we can find the exterior angle. a+x=180 because the angles are on a straight line which equals 180°
a+x=180 100+x=180 (Subtract 100 from both sides) x=80°
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