Question

Mas gmana nih

Diketahui deret aritmatika Sn= 3n2
– 2n . Carilah : a. beda barisan
b. Suku ke -5

Answer 1

Jika : Sn = - 2n
Maka :
S1 = 3() - 2(1)
= 3 - 2 = 1
S2 = 3() - 2(2)
= 12 - 4 = 8
S3 = 3() - 2(3)
= 27 - 6 = 21
S4 = 3() - 2(4)
= 48 - 8 = 40
U1 = 1
U2 = S2-S1 = 7
U3 = S3-S2 = 13
U4 = S4-S3 = 19
Dapat Disimpulkan : beda barisan = 6
U5 = 19 + 6 = 25

Answer 2

Sn = 3n² - 2n
S1 = 3.1² - 2.1 = 3 - 2 = 1
S2 = 3.2² - 2.2 = 3.4 - 4 = 12 - 4 = 8

Un = Sn - S(n-1)
U2 = S2 - S1
     = 8 - 1
     = 7
a = S1 = 1
b = U2 - U1
   = 7 - 1
   = 6
jadi beda = 6

U5 = S5 - S4
     = (3.5² - 2.5) - (3.4² - 2.4)
     = (3.25 - 10) - (3.16 - 8)
     = (75 - 10) - (48 - 8)
     = 65 - 40
     = 25
jadi suku ke-lima adalah 25

semoga membantu ya :)