Question

Why is it called vertically opposite angles?

Answer 1

Here ( in the attachment )

x and y are vertically opposite angle.
but not m and n .

Answer 2

hy
here is your answer
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Vertically Opposite Angles are the angles opposite each other when two lines ... Note: They are also called Vertical Angles, which is just another way of saying the  ...Vertical" in this case means they share the same Vertex (or corner point). In other word, we can say that two angles formed by two intersecting lines having no common arm .
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see in attachment you understand