Two or more triangles are similar if their corresponding angles are congruent. It also follows that if two triangles are similar, their corresponding sides are proportional (equal ratio). Here’s an applet you can use to teach this concept.
1. Given that DE is parallel to AC, what can we know about the angles of the two triangles?
2. How can this help you solve for sides of triangle? For example, suppose you do not know the measure of AC, how will you solve for its measure?Download Similar Triangles Applet (170 downloads)
You can give problems like the one below as applications:
In the figure, HJ = 6 cm, IF = 3 cm, JG = x cm and HI = y cm. Solve for x and y.