Question

Diketahui u3 = 18 dan u7 = 38 tentukan jumlah 24 suku pertama

Answer 1

A+2b=18
a+6b=38
berarti -4b=-20
b=5
berarti a+2(5)=18
a=8
s24=12(16+23(5)) = 12(16+115) = 12(131) = 1572

Answer 2

Cari beda
U3 = a + (3-1)b
18 = a + 2b

U7 =a + (7-1)b
38 = a + 6b

eliminasikan U7 dg U3
38 = a + 6b
18 = a + 2b  -
20 = 4b
20/4 = b
5 = b

cari suku awal
U3 = a + 2b
18 = a + 2b
18 = a + (2 x 5)
18 =  a + 10
18-10 = a
8 = a

24 suku pertama = S24

Sn = n/2 (2a + (n-1) b)
S24 = 24/2 [(2 x 8) + (24-1)5]
S24 = 12 [16 + (23 x 5)]
S24 = 12 [16 + 115]
S24 = 12 (131) = 1572