Next, let's draw a second segment QR of length b such that P, Q, and R are collinear.
Now, we draw PS of length c, such that S is not collinear to the three previous points.
To draw the fourth segment we are looking for, first we draw PS and QS. Then, we draw a line parallel to QS passing through R.
The fourth required segment is ST whose length is d. Notice that the Triangle Proportionality Theorem gives us the required proportion.
a/b = c/d âś“
Keep in mind that the segments drawn above are just an example and your diagram may vary.