If mbac = 55°, what is mdbc? a. 20°b. 80°c. 70°d. 35°

Looking at the diagram BAC is an isosceles triangle.

According to the isosceles triangle theorem. angle BAC and Angle BCA are equal.

BAC = 55
BCA = 55

Since we know two of the angles we will use the angle addition postulate to find angle ABC.

ABC + BAC + BCA = 180
ABC + 55 + 55 = 180
ABC + 110 = 180
ABC = 70

Line BD is angle bisector of angle B.

SO angle DBC is half the measure of angle ABC.

DBC = 70/2
DBC = 35

The answer is D.

Hope this helps :)