To trisect a line segment (in this case line segment AD):

Draw a circle around the midpoint of AD.

Construct the perpendicular bisector of AD and extend it until it touches the circle in two places.

Label it CF as shown.

Draw CA and DF as is shown.

Bisect CA and DF with points B and E as shown.

Draw BF and CE.

Alternately, you can use another method to trisect AD. Click on the supplied link to view this method.

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