Question

Suku ke-4 dan suku ke-7 barisan aritmatika masing-masing 19 dan 37. Jumlah 24 suku pertama adalah...

Answer 1

A+3b = 19
a + 6b = 34

b = 5
a= 4

sn = n/2 (2a+(n-1) b )
s24 = 24/2 ( 2*4 + (24-1) 5
      = 211

Answer 2

U4 = a+3b = 19
U7 = a+6b = 37 ₋
           -3b = -18
              b = 6

a+3b = 19
a+3(6)=19
a+18=19
a = 19-18 ⇒ a = 1

Sn = n÷2 {2a + (n-1)b}
S24 = 24 ÷2 {2(1)+(24-1)(6)}
       = 12 {2+23(6)}
       = 12 {2+138}
 S24 = 1680