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Book I   Proposition 48
And, since DA is equal to AB, and AC is common,
    the two sides DA, AC are equal to the two sides BA, AC;
and
    the base DC is equal to the base BC;
therefore
    the angle DAC is equal to the angle BAC. [I.8]

But
    the angle DAC is right;
therefore
    the angle BAC is also right.
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