proposition 47
in right-angled triangles, the square on the side sub.
tending the right-angle is equal to the (sum of the)
squares on the side containing the right-angle.
let ABC be a right-angled triangle having the angle
BAC a right-angle. i say that the square on BC is equal
to the (sum of the) squares on BA abd AC
for let the square BDEC have been described on BC,
and (the squares) GB and HC on AB and AC
(respectively) [prop. 1.46] and let AL have been
drawn through point A parallel to either of BD or CE
[prop. 1.31] and let AD and FC have been joined. and
since angles BAC and BAG are each right-angles, then
two straight-lines AC and AG not lying on the same
side, make the adjacent angles with some straight-line
BA, at the point A on it, (whose sum is) equal to two
right-angles. thus, CA is straigh-on to AG [prop. 1.14]
so, for the same (reasons) , BA is also straigh-on to AH
and since angle DBC is equal to FBA, for (they are)
both right-angles, let ABC have been added to both.
thus, the whole (angle) DBA is squal to the whole
(angle) FBC. and since DB is equal to BC, FB to
BA, the two (straight-lines) DB, BA are equal to the
two (straight-lines) CB, BF, respectively. and angle
DBA (is) equal to angle FBC. thus the base AD [is]
equal to the base FC and the triangle ABD is equal to
the triangle FBC [prop. 1.4] the triangle. and parallelogram BL
[is] double 9the area of triangle ABD. for they have
the same base, BD,and are between the same parallels,
BD and AL [prop. 1.4] and square GB is double
(the are) of triangle FBC. for again they have the same
base , FB, and are between the same parallels, FB and
GC [prop. 1.4] . [and the doubles of equal things area equal to one another].
thus, the whole square
BDEC is equal to the (sum of the) two squares GB and
HC, and the square BDEC is described on BC, and
the (squares) GB and HC on BA and AC (respectively).
thus, the squares on the side BC is equal to the (sum of the)
squares on the side BA and AC.