Question: two vectors are perpendicular if the dot product?
Answer: If two vectors are perpendicular, then their dot-product is equal to zero. The cross-product of two vectors is defined to be A×B = (a2_b3 - a3_b2, a3_b1 - a1_b3, a1_b2 - a2*b1). The cross product of two non-parallel vectors is a vector that is perpendicular to both of them.