Answer
a) Vertical Angles Theorem
b) $\angle1\cong\angle6$
c) Vertical Angles Theorem
d) Transitive Property of Congruence
Work Step by Step
$\angle3$ and $\angle6$ are vertical angles.
Since $\angle1$ and $\angle6$ are both congruent to $\angle3$, they are congruent to each other.
$\angle1$ and $\angle4$ are vertical angles.
Since $\angle4$ and $\angle6$ are both congruent to $\angle1$, they are congruent to each other.
