discretebook-fall2015.pdf, page 53, exercise 5d

[jonjitsu]

Counting how many trapezoids can be made given the 2 rows of 7 dots.

Perhaps it would help to add whether you consider a parallelogram as a trapezoid or not. Before attempting this exercise I checked a few resources and there seems to be 2 camps. One camp states that a trapezoid has exactly 1 set of parallel lines which would exclude parallelograms. The other camp states that a trapezoid has at least 1 set of parallel lines and that a parallelogram is a trapezoid.

You could even make 2 questions out of it based on definition.

Anyways this is more of a sugesstion than an issue.

[oscarlevin]

Good point. I never even considered the possibility of a parallelogram not being a trapezoid. I've put a footnote in to this effect and added a part as you suggest. I still need to go back and check the solution (perhaps there is a nicer way to think of this instead of just subtracting the parallelograms).