hitunglah jumlah 18 buah bilangan ganjil

1+3+5+7+9+11+13+15+17+19+21+23+25+27+29+31+33+35=324

bilangan:1,3,5,7,9,11,13,15,17,19,21,23,25,27,29,31,33,35
a=1 b=2
un=a+(n-1)b
35=1+(n-1)2
35=1+2n-2
35=2n-1
35+1=2n
2n=36
n=36/2
n=18
sn=1/2×n(a+un)
    =1/2×18(1+35)
    =9(36)
    324