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Book I   Proposition 46
And, since
    the straight line AD falls upon the parellels AB, DE,
    the angles BAD, ADE are equal to two right angles. [I.29]
But
    the angle BAD is right;
therefore
    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.

Therefore ADEB is also right-angled.
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