Question

Dari suatu deret diketahui sn = 3n^2 - 15n. Nilai Un = 0 untuk n = ....

a.1
b.2
c.3
d.4
e.5

Answer 1

S1= 3 x1² - 15x1= 3- 15 = -12
S2= 3x2² - 15x2 = 12 - 30 = -18

a = -12
u2 = S2 - a = -18 +12  = -6
b = -6 - (-12) = 6

un= a +(n-1)b
 0  = -12 + (n-1)6
12 = 6n - 6
18= 6n
n= 3