Question 391009
Since AD || BC, angle 1 = angle 3, so you can replace the "3" with a "1." Also, since angle 2 = angle 1 (already given), AB = BC, and since AB = CD and BC = AD (the parallel sides of a parallelogram are congruent), all four sides are congruent to each other and it's a rhombus.


I almost never use the two-column proof when writing solutions to proof problems...it helps you back up each statement with a reason, but virtually no mathematicians use the two-column proof when explaining a proof(e.g. in a paper).