Theorems about Similar Triangles

1. The Side-Splitter Theorem

To show this is true, draw the line BF parallel to AE to complete a parallelogram BCEF:

Triangles ABC and BDF have exactly the same angles and are therefore similar (Why? See the section called AA on the page How To Find if Triangles are Similar.)

• Side AB corresponds to side BD and side AC corresponds to side BF.
• Therefore AB/BD = AC/BF
• But BF = CE
• Therefore AB/BD = AC/CE

The Angle Bisector Theorem

To show this is true, we can label the triangle like this:

• Angle BAD = Angle DAC = x°
• Angle ADB = y°
• Angle ADC = (180 - y)°

By the Law of Sines in triangle ABD:

sin x°/BD = sin y°/AB

Therefore AB × sin x° = BD × sin y°

Therefore:

AB/BD = sin y°/sin x°

By the Law of Sines in triangle ACD:

sin x°/DC = sin(180 - y)°/AC

Therefore AC × sin x° = DC × sin(180 - y)°

Therefore AC/DC = sin(180 - y)°/sin x°

But sin(180 - y)° = sin y°

Therefore:

AC/DC = sin y°/sin x°

Combining AC/DC = sin y°/sin x° with AB/BD = sin y°/sin x° gives:

AC/DC = sin y°/sin x° = AB/BD

Therefore AB/BD = AC/DC

In particular, if triangle ABC is isosceles, then triangles ABD and ACD are congruent triangles

And the same result is true:

AB/BD = AC/DC

3. Area and Similarity

Example:

These two triangles are similar with sides in the ratio 2:1 (the sides of one are twice as long as the other):

What can we say about their areas?

The answer is simple if we just draw in three more lines:

You can see that the small triangle fits into the big triangle four times.

So when the lengths are twice as long, the area is four times as big

Therefore the ratio of their areas is 4:1

We can also write 4:1 as 2²:1

The General Case:

Triangles ABC and PQR are similar and have sides in the ratio x:y

We can find the areas using the formula from Finding the area of a triangle that has no right angle.

ABC's Area = ½bc sin A

PQR's Area = ½qr sin P

And we know the lengths of the triangles are in the ratio x:y

Therefore q/b = y/x, so: q = by/x

and r/c = y/x, so r = cy/x

Also, since the triangles are similar, angles A and P are the same:

A = P

We can now do some calculations:

So we end up this ratio:

Area of triangle ABC : Area of triangle PQR = x² : y²