dari barisan aritmatika diketahui u3 =18 dan u7= 38.jumlah 24 auku pertama adalah mohon dijawab.

U3 = a + (n-1) b
18 = a + (3-1) b
18 = a + 2b... (pers. 1)

U7 = a + (n-1) b
38 = a + (7-1) b
38 = a + 6b... (pers. 2)

Eliminasi a
18 = a + 2b
38 = a + 6b
________-
-20 = -4b
b = -20/-4
b = 5

b = 5 disubstitusikan ke pers. 1
18 = a + 2b
18 = a + 2.5
18 = a + 10
18 - 10 = a
8 = a

a = 8, b = 5
S24 = 1/2 . n (2a + (n-1) b)
= 1/2 . 24 (2.8 + (24-1) 5)
= 12 (16 + 115)
= 12 . 131
= 1572