Draw a line across a triangle parallel to one side and it cuts the other two sides in the same ratio. That gives you a proportion with one unknown, and cross-multiplying finishes it.
In triangle ABC, DE is parallel to BC. AD=4, DB=6, AE=8. Find EC
AD over DB equals AE over EC.
4 over 6 equals 8 over EC.
Cross-multiply: 4 times EC is 48.
The answer is 12.
Where students go wrong
Using the whole side where a piece belongs. The theorem matches the two pieces of one side against the two pieces of the other, so AD goes with AE and DB goes with EC. Putting the full length AB into the proportion gives the wrong answer.
Also called side-splitter theorem, triangle proportionality theorem or parallel line in a triangle.
Questions about these worksheets
Yes. Every triangle proportionality sheet is free to print and free to download as a PDF. There is no account to make, no email to hand over and no limit on how many you take.
Yes. The answer key prints on a second page, and you can choose whether it shows each question beside its answer or just the answers in a list.
Yes. Print straight from the page, or download the PDF and print that. The sheet is laid out for paper rather than squeezed off a screen, so there is room to work under each question, and the answer key comes out on its own page.
Geometry is usually taken around 9th or 10th grade, though schools vary and plenty of students meet it earlier or later. Pick the difficulty rather than the grade: the easy level suits a first lesson on it, the hard level suits review before a test.
Yes, as many as you like. The questions are built fresh each time rather than picked from a fixed set of files, so pressing Generate gives a new sheet. That is what you want for a second class, a retake, or two students sitting next to each other.
That is the closest comparison, and Kuta Software is good software. It is also paid software you install on a computer. These worksheets run in a browser tab for free, with no account and nothing to install. Like Kuta, the triangle proportionality questions are generated when you ask for them rather than pulled from a fixed set of files, so two classes never get the same sheet, and the answer key comes with it.
Yes. There are two levels, and you can tick more than one for a sheet that mixes them, in which case the questions come out easiest first. The harder levels are not just larger numbers: they ask for something the easy ones do not.
Using the whole side where a piece belongs. The theorem matches the two pieces of one side against the two pieces of the other, so AD goes with AE and DB goes with EC. Putting the full length AB into the proportion gives the wrong answer.