Question

Find first differences for the sequence in order from a_1 to a_5. Determine whether or not the series is quadratic or not.(I used _ as a sign for a subscript)

-1, -3, -1, 5, 15

A. 2, 2, 6, 10; not quadratic

B. 2, 2, 6, 10; quadratic

C. -2, 2, 6, 10; not quadratic

D. -2, 2, 6, 10; quadratic

Answer 1

well, just look at the differences:

1st differences: -2 2 6 10
2nd differences: 4 4 4

Looks quadratic to me.