Question

Mohon bantuannya

Dari barisan aritmetika U3=14 dan U2=26 maka jumlah 18 U1 adalah?

Pake caranya, jgn asal-asalan

Answer 1

Un = a + (n-1)b

a = u1

Un = U1 + (n-1)b
U3 = U1 + (3-1)b
= U1 + 2b

U1 + 2b = 14

U2 = U1 + (2-1)b
= U1 + b

U1 + b = 26

U1 + 2b = 14
U1 + b = 26__-
b = -12

U1 + b = 26
Uq - 12 = 26
U1 = 26 + 12
U1 = 38

Sn = 1/2 n (2U1 + (n-1)b)
S18 = 1/2 18 (2.38 + (18-1)(-12))
= 9 (76 -204)
= 9 (-128)
= -1.152

Answer 2

Cari beda :
( 14 - 26 ) : ( 3 - 2 ) = -12

Cari awal :
14 = a + 2b
14 = a - 24
  a = 24 + 14
  a = 38

Maka, jumlah 18 suku pertama :
S₁₈ = 18 : 2 ( 2 × 38 + ( 18 - 1 ) × -12 )
      = 9 ( 76 + 17 × -12 )
      = 9 × -128
      = -1.152