A (complete) quadrilateral is a set of 4 lines, no 3 concurrent, and the 6 points determined by the lines If a, b, c, d are the 4 lines then a b and c d, a,c and b d, and a d and b c are pairs of opposite vertices The lines joining opposite pairs of vertices are called diagonal lines of the quadrilateral
a shape with four sides -- "If two sides of a quadrilateral are parallel " (250)
A quadrilateral is a 4-sided figure: It can be any shape, as long as it has four sides A square is a quadrilateral Now here is a problem for you: Name another quadrilateral figure