McGraw Hill
Glencoe: Online Study Tools: Self Check Quiz on Perpendicular Vector
Learners display understanding of advanced mathematical concepts in Geometry. The online quiz assesses the scholars on perpendicular vectors.
Wolfram Research
Wolfram Math World: Perpendicular Vectors
Come to this site to find the transformational matrix to find the vector rotated 90 degrees from the original. The site also states the special relationship between two perpendicular vectors when using the dot product.