Similar Triangles

Similar means same shape, different size. See why two triangles are similar, then use the equal ratios of matching sides to find a missing length.

AA similarity test
Two equal angles
Similar triangles
△ABC is similar to △PQR
Matching sides are in the same ratio. Write a pair you know over a pair with the unknown, then cross-multiply to find x.
Matching sides in proportion
Cross-multiply and solve
Test
Known side AB AB = 6