Question

Jumlah n suku pertama suatu deret aritmatika ditentukan oleh rumus Sn = 2n^2 - 6n . Beda deret tersebut adalah...
a. -4
b. 3
c. 4
d. 6
e. 8

Answer 1

Verified answer

S1 = 2(1)²-6(1) = 2-6 = -4 -> a
S2 = 2(2)²-6(2) = 8-12= -4
u2 = s2-s1 = -4-(-4) = -4+4 = 0
b= u2-u1 = 0-4 = -4

Answer 2

Sn = 2n^2 - 6n

S1 = U1
2(1)^2 - 6(1) = U1
2 - 6 = U1
-4 = U1

S2 = U1 + U2
2(2)^2 - 6(2) = -4 + U2
8 - 12 = -4 + U2
0 = U2

Beda = U2 - U1 = 0 - (-4) = 4

Cara cepat :
Sn = pn^2 + qn ==> beda = 2p
Sn = 2n^2 - 6n ==> beda = 2(2) = 4