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Vertical Angles are the angles opposite each other when two lines cross
In this case, “Vertical” means they share the same corner point (Vertex), not the usual meaning of up-down.
a° and b° are vertical angles.
Vertical Angles are Congruent Angles
The interesting thing here is that vertical angles are congruent angles:
a° = b°
they are equal
Find angles a°, b° and c° below:
Because a° is vertically opposite 100°, it must also be 100°
A full circle is 360°, so that leaves 360° − 2×100° = 160°
Angles b° and c° are also vertical angles, so must be equal, which means they are 80° each.
Answer: a = 100°, b = 80° and c = 80°.