Question

Look at the quadrilateral shown below.



Terra writes the following proof for the theorem, “If the diagonals of a quadrilateral bisect each other, the quadrilateral is a parallelogram.”

Terra’s proof

Answer 1

I know this is 3 months late, but if anyone else comes across this, I thought I'd just post to help them. :)
Statement 3 is incorrect; they are equal because they are vertically opposite, not because they are supplemental angles.

Answer 2

@turtlemaroon , thannks!!