Question

Rumus jumlah n suku pertama suatu deret aritmatika adalah sn = 1/6 n (n+2) maka carilah a, beda, dan u7

Answer 1

Sn1 = 1/6 x 1 (1+2)
= 3/6 = 1/2
a = sn1 = 1/2
sn2 = 1/6 x 2 (2+2)
= 8/6
b = sn2-sn1 = 8/6-3/6 = 5/6
sn7 = 1/6 x 7 (7+2)
= 21/2

Answer 2

Rumus asala => Sn = 1/2n (a+Un) => Un = a+(n-1)b
Sn = 1/2n (2a+bn-b)
jadi 1/6n (nt2) = 1/2n (2a+bn-b)
Kita umpamakan n =1 maka,
1/2(2a+b-b) = 1/6(1+2) => a =1/2
Jadi S2 = 1/6×5 = 5/6
maka S2 = Sn, 5/6 = 1/2×2 (2×1/2+2b-b) => b = 5/6
U7 = 1/2+(7-1)×5/6 = 11/2

Mohon koreksi. Thx