Kak, tolong bantuin nomor nomor 15 jugaa yaa heee​

U1 = a = 1/3
r = U2/U1 = 1/1/3 = 3

Sn = a(rⁿ – 1)/ r – 1

S1 = 1/3( 3ⁿ – 1)/ 3-1

S1 = 1/6 (3ⁿ – 1)