jumlah 13 suku pertama dari bilangan ganjil adalah

U13 = 13/2 (2 x 1 + (13-1) 2)
= 13/2 (26)
= 169

bilangan ganjil : 1,3,5,...
a=1
b= u2-u1 = 3-1 =2

[tex]sn = \frac{n}{2} (2a + (n - 1)b)[/tex]
[tex]s13 = \frac{13}{2} (2 \times 1+(13 - 1\times 2)) \\ s13 = \frac{13}{2} (2+(12 \times 2))[/tex]
[tex]s13 = \frac{13}{2} (2+24) \\ s13 = \frac{13}{2} (26)[/tex]
S13 = 169