Question

Pada Barisan aritmetika u3=18 , u7=38 jumlah 24 suku pertama adalah ?

Answer 1

Cari beda :
( 38 - 18 ) : ( 7 - 3 ) = 5

Cari awal :
18 = a + 2b
18 = a + 10
  a = 8

Jumlah 24 suku pertama :
S₂₄ = 24 : 2 ( 2 × 8 + ( 24 - 1 ) 5 )
S₂₄ = 12 ( 16 + 115 )
S₂₄ = 12 × 131
S₂₄ = 1.572

Answer 2

Un = a + (n-1) b
U7 = a + 6b
38 = a + 6b

U3 =a + 2b
18 = a + 2b

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

18 = a + 2b
18 = a + 10
8 = a

Sn = 1/2n (2a+ (n-1)b)
s24 = 12 (16 + 23.5)
s24 = 12. 131
s24 = 1572

semoga membantu^^