Similar Triangle Theorems and Rules

Definition of Similarity

Two triangles are similar if their corresponding angles are equal and their corresponding sides are proportional:

$$\mathrm{△}ABC\sim \mathrm{△}DEF\text{if and only if}\{\begin{array}{l}\mathrm{\angle }A=\mathrm{\angle }D,\\ \mathrm{\angle }B=\mathrm{\angle }E,\\ \mathrm{\angle }C=\mathrm{\angle }F,\\ \frac{AB}{DE}=\frac{BC}{EF}=\frac{AC}{DF}\end{array}$$ Where $\sim$ is the symbol of similarity of two triangles.

Similarity Postulates

1. Angle-Angle (AA) Similarity

If two angles of one triangle are equal to two angles of another triangle, the triangles are similar since the third angle automatically follows from angle sum property of Triangles.

Example:
Given $\mathrm{\angle }A=\mathrm{\angle }D$ and $\mathrm{\angle }B=\mathrm{\angle }E$,
$\Rightarrow \mathrm{△}ABC\sim \mathrm{△}DEF$

2. Side-Angle-Side (SAS) Similarity

If two sides of one triangle are proportional to two sides of another triangle and the included angles are equal, the triangles are similar.

Example:
${\displaystyle \frac{AB}{DE}}={\displaystyle \frac{AC}{DF}}$ and $\mathrm{\angle }A=\mathrm{\angle }D$
$\Rightarrow \mathrm{△}ABC\sim \mathrm{△}DEF$

3. Side-Side-Side (SSS) Similarity

If all three sides of one triangle are proportional to the corresponding sides of another triangle, the triangles are similar.

Example:
${\displaystyle \frac{AB}{DE}}={\displaystyle \frac{BC}{EF}}={\displaystyle \frac{AC}{DF}}$
$\Rightarrow \mathrm{△}ABC\sim \mathrm{△}DEF$

Key Theorems

Basic Proportionality Theorem (Thales' Theorem)

If a line is drawn parallel to one side of a triangle, it divides the other two sides proportionally.

Example:
If $DE\parallel BC$ in $\mathrm{△}ABC$,
then ${\displaystyle \frac{AD}{DB}}={\displaystyle \frac{AE}{EC}}$.

Right Triangle Similarity

The altitude to the hypotenuse of a right triangle creates two smaller triangles similar to each other and to the original triangle.

Example:
In right $\mathrm{△}ABC$ with altitude $CD$:
$\mathrm{△}ABC\sim \mathrm{△}ACD\sim \mathrm{△}CBD$ (all three triangles are similar to each other).

Midline Theorem

The segment connecting the midpoints of two sides of a triangle is parallel to the third side and half its length.

Example:
If $D$ and $E$ are midpoints of $AB$ and $AC$,
then $DE\parallel BC$ and $DE={\displaystyle \frac{1}{2}}BC$.

Important Properties

Ratio of Areas

The ratio of the areas of two similar triangles is the square of the ratio of their corresponding sides.

$${\displaystyle \frac{\text{Area of }\mathrm{△}ABC}{\text{Area of }\mathrm{△}DEF}}={\left({\displaystyle \frac{AB}{DE}}\right)}^2$$

Angle Bisector Theorem

An angle bisector divides the opposite side into segments proportional to the adjacent sides.

Example:
If $AD$ bisects $\mathrm{\angle }A$ in $\mathrm{△}ABC$,
then ${\displaystyle \frac{BD}{DC}}={\displaystyle \frac{AB}{AC}}$.

More References and Links to Geometry Problems

• Geometry Tutorials, Problems and Interactive Applets
Similar Triangles Examples and Problems with Solutions