Dik : barisan aritmatika 2,5,8,11 Dit : jumlah 15 suku pertama

Sn=1/2n (2a+(n-1)b) Sn=1/2×15 (2.2+(15-1)3) =7,5×(4+(14)×3) =7,5x(4+42) =7,5×46 =345

U₁ = 2

b = U₂ - U₁
b = 5 - 2[tex]~[/tex]
b = 3

Sn = n/2 ( 2a + ( n - 1 ) b )
S₁₅ = 15/2 ( 2(2) + ( 15 - 1 ) 3 )
S₁₅ = 15/2 ( 4 + 14(3) )
S₁₅ = 15/2 ( 4 + 42 )
S₁₅ = 15/2 ( 46 )
S₁₅ = 15 ( 23 )
S₁₅ = 345