Question

Jumlah n suku pertama suatu deret didefinisikan sebagai Sn= 3n² - 4n. Jika Un adalah suku ke-n, maka U10 = ...?

Answer 1

U10=S10-S9

S10=3x10²-40
S10=300-40=260

S9=3x9²-36
S9=243-36
S9=207

U10=260-207=53

Answer 2

Sn = 3n²  - 4n

S1 = U1 = a
S1 = 3(1)² - 4(1)
S1 = 3 - 4
S1 = -1
maka a = -1

S2 = U1 + U2 =  a + a + b
S2 = 3(2)² - 4(2)
S2 = 12 - 8
S2 = 4
maka U2 = S2 - U1 = 4 - (-1) = 5
maka beda = S2 - 2a = 4 - (2 x -1) = 4 - (-2) = 6

jadi a = -1, b = 6

U10 = a + 9b
U10 = -1 + ( 9 x 6)
U10 = -1 + 54 = 53