common Difference fed=r
Sum=(a+r1) +(a+r2) + (a+r3) + (a+r4) + (a+r5) + ... + (a+rn)
where r1 = 0
r2=-r
r3=3r
r4=-6r
r5=10r
r6=-15r
................
Thus Sum = a + (a-r) + (a+3r) + (a-6r) + (a+10r) +.....
Example= 1 + 5 - 11 + 25 - 39 +61 -........
where 1st term= a =1
common difference r =- 4 ;