Due to the Triangle Angle Sum Theorem, we know that the sum of the interior angles of a triangle equals 180 degrees, therefore,
180 = m<1 + m<2 + m<3
180 - m<3 = m<1 + m<2
But, we also know that m<4 + m<3 = 180 degrees.
180 = m<3 + m<4
180 - m<3 = m<4
Both m<4 and m<1 + m<2 equals 180 - m<3
m<4 = m<1 + m<2
