The defect of triangle △ABC is defined as Note that Theorem 11.3.1 states that the defect of any triangle in a neutral plane has to be nonnegative. Let △ABC be a nondegenerat...The defect of triangle △ABC is defined as Note that Theorem 11.3.1 states that the defect of any triangle in a neutral plane has to be nonnegative. Let △ABC be a nondegenerate triangle in the neutral plane. defect(△ABC)=defect(△ADC)+defect(△DBC). Suppose that ABCD is a rectangle in a neutral plane; that is, ABCD is a quadrangle with all right angles.