-
20 21 19. 2 21 11 10 21 10 21
1+2+3+4 18+19+20= 1+20 + 2+19 10+11 =210
210
19
= 19+1 *19/2=190
1 7 20 1
1
20+1 *20/2- 6+1 *6/2=210-21=189 7
d n sn=na1+n n-1 d/2 sn=n a1+an /2