Question 1191752
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In an isosceles trapezoid, the two angles on each base are congruent, so in this trapezoid angles A and B are congruent, and angles C and D are congruent.<br>
In an isosceles trapezoid, the measures of the angles on opposite bases are supplementary (add to 180 degrees).  So in this trapezoid the measures of angles B and D add to 180 degrees.<br>
Simple arithmetic....<br>