Euclid's Elements: Book I: Proposition 11
Proposition 11: To draw a straight line at right angles to a given straight line from a given point on it.
Let AB be the given straight line, and C the given point on it.
Thus it is required to draw from the point C a straight line at right angles to the straight line AB.
Let a point D be taken at random on AC;
let CE be made equal to CD; I. 3 on DE let the equilateral triangle FDE be constructed, I. 1 and let FC be joined;
I say that the straight line FC has been drawn at right angles to the given straight line AB from C the given point on it.
For, since DC is equal to CE, and CF is common, the two sides DC, CF are equal to the two sides EC, CF respectively; and the base DF is equal to the base FE; therefore the angle DCF is equal to the angle ECF; I. 8 and they are adjacent angles.
But, when a straight line set up on a straight line makes the adjacent angles equal to one another, each of the equal angles is right; Def. 10 therefore each of the angles DCF, FCE is right.
Therefore the straight line CF has been drawn at right angles to the given straight line AB from the given point C on it.