Find the measure of the exterior angle X:
125

x=80°

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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