suatu barisan aritmatika mempunyai suku ke-4 = 15 dan suku ke-7 = 24 jumlah 14 suku dari barisan tersebut adalah pakai cara

[tex]u4 = a + 3b \\ 15 = a + 3b \\ \\ u7 = a + 6b \\ 24 = a + 6b \\ \\ 24 = a+ 6b \\ 15 = a + 3b \\ - - - - - - - \\ 9 = 3b \\ b = 3 \\ \\ 15 = a + 3(3) \\ 15 - 9 = a \\ a = 6 \\ \\ sn= \frac{n}{2} (2a + (n - 1)b) \\ s14 = \frac{14}{2} (12 + 39) \\ s14 = 7(51) \\ s14 = 357[/tex]

U4 ----> a + 3b = 15
U7 ----> a + 6b = 24
             ---------------- -
                  -3b = -9
                     b = -9/-3
                        = 3
a + 3b = 15
a + 3(3) = 15
a = 15 - 9
   = 6

U₁₄ = a + 13b
      = 6 + 13(3)
      = 45

Sn   = n/2 (a + Un)
S14 = 14/2 (6 + 45)
        = 7 (51)
        = 357