dari barisan aritmatika diketahui suku ke-2=18 dan suku ke-7=38.jumlah 24 suku pertama adalah

jawab

U7 = 38
U2 = 18
(7-2)b = 38 -18
5b = 20
b = 4

u2 = 18
a  = 18 - b  = 14

Sn = n/2 ( 2a  + (n-1) b)
S24 = 12 (28 + 23(4)) = 12 (120)
s24 = 1.440

U2 = 18 ==> a+b = 18
U7 = 38 ==> a+6b = 38

a+6b = 38
a+ b = 18
________ -
5b = 20
b = 4
a+b = 18
a+4 = 18
a = 18-4
= 14

Sn = n/2 (2a+(n-1)b)
S24 = 24/2 (2.14+(24-1).4)
= 12 (28+23.4)
= 12 (28+92)
= 12 ( 120)
= 1440