versión impresa ISSN 1665-5826
PEREZ JUAREZ, Ángel. Perpendicular lines. Educ. mat [online]. 2010, vol.22, n.3, pp. 143-148. ISSN 1665-5826.
A proof of the orthogonality condition between two lines without using trigonometric functions is presented. The alternative proof consists in developing a second degree equation and working with quadratic equations properties, such as the relationship between the number of solutions and the discriminant sign. This approach shows as well the usefulness of some of the properties of a quadratic polynomial.