extremities of the line segment are unequal. Figure 3-15 shows that line AC is equal to BC and line AD

is equal to BD, but line AE is not equal to BE.

respective sides are either parallel or perpendicular to each other.

a. Theorem 1. If two angles have their sides respectively parallel, the angles are either equal or

supplementary, as shown in Figure 3-16. Line C1C11 is parallel with AC, and line A1B1 is parallel with

AB. Angle B1A1C1 is equal to BAC, and angle B1A1C11 is the supplement of B1A1C1; therefore, angle

B1A1C11 is also the supplement of BAC.

b. Theorem 2. If two angles have their sides respectively perpendicular, the angles are either

equal or supplementary, as shown in Figure 3-17. Line C1C11 is perpendicular to AC, and line A1B1 is

perpendicular to AB. Angle B1A1C1 is equal to BAC, and angle B1A1C11 is the supplement of B1A1C1;

therefore, angle B1A1C11 is also the supplement of BAC.