if a trapezoid has a line of symmetry and one of its angles measures 100 degrees, what is the measure of each of its other angles
The answer is 80°.
The four interior angles of a trapezoid, like all four sides polygons, add up to 360°. Because of the line of symmetry, two of the angles are 100°.
That leaves 160° left for the other two angles and 160° divided by 2 is 80°.