Lines When two straight lines intersect, the angles on either side of the intersecting line are called supplementary angles. The total of the two angles is 180 degrees. In the figure below, the two angles above line L1 total 180 degrees, so angle b = 180 - 110 = 70 degrees. Supplementary angles total 180 degrees Also angles that are diagonally across from each other are equal. They are called vertical angles. In the figure above angle c = angle b. And, angle d = 110, since d is the vertical angle to the 110 degree angle.
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