Answer:
1. DB = 10 as diagonals of isosceles trapezoid are congruent
2. ∠DAB and ∠BCD are supplementary angles
- m∠BCD = 180° - 120° = 60°
3. ∠BCD is congruent with ∠ADC as trapezoid is isosceles
4. AB = 15, DC = 25,
- EB = 1/2(AB + DC) as midline
- EB = 1/2(15 + 25) = 20
5. m∠ADC = 2x - 10, m∠ABC = 3x + 40
These are supplementary, find x first:
- 2x - 10 + 3x + 40 = 180
- 5x + 30 = 180
- 5x = 150
- x = 30
- m∠BCD = m∠ADC = 2*30 - 10 = 50°
