Which pair of angles are alternate interior angles?

Answer 1

_\_________

.a\b

_c\d________

.....\

When a line crosses 2 lines, 8 angles are formed.

Four are exterior angles - outside the 2 lines, and four are interior angles.

These are labelled a, b, c, d in the diagram.

a & d are alternate interior angles because they alternate from one side

of the intersecting line to the other; b & c are also alternate interior angles.

They are also known as "Z-angles" because the top parallel line, the transversal and the bottom parallel line which define the two angles for the letter Z (or a distorted version of it).

If angle a = angle d (in which case angle b = angle c as well), the 2 lines drawn horizontally are parallel.

If alternate interior angles are equal, the 2 lines are parallel.

OR If you know the lines are parallel, then alternate interior angles must be equal.

Not the greatest diagram; please ignore the ... but even a lousy diagram helps.

And no, you don't use lower case letters for angles but there shouldn't be any confusion.