Given: 1 || m; 21 = 23 prove: plla. complete the missing parts of the paragraph proof pg we know that angle 1 is congruent to angle 3 and that line lis parallel to line m because we see that is congruent to by the alternate interior angles theorem. therefore, angle 15 congruent to angle 2 by the transitive property so we can conclude that lines p and q are parallel by the