Proving a Quadrilateral Is a Parallelogram Answers

Parallelogram R S T U is shown. Diagonals are drawn from point R to point T and from point U to point S and intersect at point V. The length of line segment U V is (x minus 3) meters and the length of line segment V S is (3 x minus 13) meters.

Given: AB ≅ CD and AD ≅ BCProve: ABCD is a parallelogram. Statements Reasons1.AB ≅ CD;AD ≅ BC1.given2.AC ≅ AC2.reflexive property3.△ADC ≅ △CBA3.?4.∠DAC ≅ ∠BCA; ∠ACD ≅ ∠CAB4.CPCTC5.∠DAC and ∠BCA are alt. int. ∠s;∠ACD and ∠CAB are alt. int. ∠s5.definition of alternate interior angles6.AB ∥ CD; AD ∥ BC6.converse of the alternate interior angles theorem7.ABCD is a parallelogram7.definition of parallelogramWhat is the missing reason in step 3?