If m ABC = 80° what is mzDBC? Justify your reasoning.
• Using the Angle Addition Postulate, 20 + m2DBC = 80. So, mzDBC = 60° using the subtraction property of equality.
Using the Angle Addition Postulate, 20 + 80 = m_DBC. So, m_DBC = 100° using the addition property of equality.
Using the addition property of equality, 20 + 80 = 100, so m_DBC = 100°.
Using the subtraction property of equality, 80 - 20 = 60, so m2DBC = 60°.