Sum=a +(a+r) + (a+r1) + (a+r2) + (a+r3) + ... + (a+rn) where r1=2r
r2=3r
r4=4r
rn=nr
Thus Sum= a +(a+r) +(a+3r) +(a+6r) + (a+10r)+(a+15r) +......
supposing tn is the nth term, if t2-t1=r then
t3-t2=2r
t4-t3=3r
t5-t4=4r
tn-t(n-1)=(n-1)r
Example--- 1 + 3 + 7 + 13 + 21 + 31 + 43 +.....
where sum(r(n-1)) =0 + r +2r +3r +....(n-1)r;