Vertically Opposite Angles Definition
Vertically Opposite Angles are the angles opposite each other when 2 lines cross
Vertical in this case means they share the same corner point (vertex), not the usual meaning of up-down.
a and c are vertically opposite angles. b and d are also vertically opposite angles.
The interesting thing here is that vertically opposite angles are equal:
a = c
b = d
(in fact they are congruent angles)
Find angles a, b and c below:
Because b° is vertically opposite 60°, it must also be 60°
So, b = 60°
A full circle is 360°, so that leaves 360° − 2×60° = 240°
Angles a and c are also vertically opposite angles, so must be equal, which means they are 120° each.
a = 120°, b = 60° and c = 120°.