U3 = 18 U7 =38 Tentukan jumlah 24 suku pertama dari barisan tersebut

Dik : U3 = 18
U7 = 38

Dit : S24?

Peny:

U3 = a + 2b = 18
U7 = a + 6b = 38
______________
-4b = -20
b = 5

a + 2b = 18
a + 2(5) = 18
a + 10 = 18
a = 18 - 10
a = 8

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

Semoga Membantu

Un = a + (n-1)b
U3 = 18 ==> a + 2b = 18
U7 = 38 ==> a + 6b = 38
-4b = -20 ==> b = 5 berarti a = 8

Sn = n/2 (2a+(n-1)b)
S24 = 12 (16 + 115)
S24 = 12 (131)
S24 = 1572

jumlah 24 suku pertama sama dengan 1572