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Book I   Proposition 45
And, since
    the straight line HG falls upon the parallels KM, FG,
    the the alternate angles MHG, HGF are equal to one another. [I.29]

    Let the angle HGL be added to each;
therefore the angles MHG, HGL are equal to the angles HGF, HGL. [C.N.2]
    But the angles MHG, HGL are equal to two right angles [I.29];
therefore
    the angles HGF, HGL are also equal to two right angles. [C.N.1]

Therefore
    FG is in a straight line with GL. [I.14]
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