When is a quadrilateral a parallelogram? a. when opposite sides are parallel but not congruent. b. when opposite sides are congruent but not parallel. c. when opposite sides are congruent and parallel. d. none of the above

C. when opposite sides are congruent and parallel

Sum i=1 to 12 of (3i-1):

2+5+...+35

12 terms; sum of first and last terms is 37.

Sum = (number of terms) times (average of all the terms)

For an arithmetic sequence, the average of all the terms is the average of the first and last terms:

Sum = (number of terms) times (average of first and last terms)