The sides of the octagon are equal, and the alternate angles coincident.
That is to say, “If alternate angles are unequal, do the lines meet?”
Euclid introduced the subject by the proposition that, if alternate angles are equal, the lines are parallel.
Frustules oblong or quadrate, adnate in filaments, attached by alternate angles and finally separating.
We may now state Prop. 16 thus:—If two straight lines which meet are cut by a transversal, their alternate angles are unequal.
And “if the lines are parallel, are alternate angles necessarily equal?”
alternate angles Two angles formed on opposite sides of a line that crosses two other lines. The angles are both exterior or both interior, but not adjacent. |