H
E
E
L
E
M
E
N
T
S
let the straight line GK fall upon them.
Then, since
the straight line GK has fallen on the parallel straight lines AB, EF,
the angle AGK is equal to the angle GHF. [I.29]
Again, since
the straight line GK has fallen on the parallel straight lines EF, CD,
the angle GHF is equal to the angle GKD. [I.29]
