where the absolute value of numbers are in AP and the alternate terms change sign. They are of the type a + (a + r) + (a - r) + ( a + 2r ) + ( a - 2r )

Here first term is substracted from the second term to find common difference( c.d).

Test With--

1) 3 - 6 + 12 - 15 + 21 - 24 where 1st term is 3, c.d= - 9 ; no. of terms = 6

2) 3 - 6 + 12 - 15 + 21

U can construct the series by putting 1st term, c.d, and number of terms 1, 2, 3, 4 etc.