|Book I Proposition 46|
the straight line AD falls upon the parellels AB, DE,
the angles BAD, ADE are equal to two right angles. [I.29]
the angle BAD is right;
the angle ADE is also right.
And in parallelogramic areas the opposite sides and angles are equal to one another [I.34]; therefore
each of the opposite angles ABE, BED is also right.
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