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S		 Book VI   Proposition 27
    Let AB be a straight line and let it be bisected at C;
    let there be applied to the straight line AB the parallelogram AD deficient by the parallelogramic figure DB described on the half of AB, that is CB;
I say that
    of all the parallelograms applied to AB and deficient by parallelogramic figures similar and similarly situated to DB, AD is greatest.

For
    let there be applied to the straight line AB the parallelogram AF
deficient by
    the parallelogramic figure FB similar and similarly situated to DB;
I say that
    AD is greater than AF.
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