Given two unequal straight lines, to cut off from the greater a straight line equal to the less.
Let AB, C be the-two given unequal straight lines, and let AB be the greater of them. Thus it is required to cut off from AB the greater a straight line equal to C the less. At the point A let AD be placed equal to the straight line C; [I. 2] and with centre A and distance AD let the circle DEF be described. [Post. 3]
(Being) what it was required to do.