Question

Tentukan banyak suku dari barisan 56 + 51 + 46 + 41 +... = - 36

Answer 1

A = 56
b = 51 - 56 = - 5
Un = a + (n - 1)b
    = 56 - 5(n - 1)
    = 61 - 5n
Sn = (n/2)(a + Un)
- 36 = (n/2)(56 + 61 - 5n)
- 72 = - 5n² + 117n
5n² - 117n - 72 = 0
(5n + 3)(n - 24) = 0
ambil n > 0
maka n = 24

Answer 2

A= 56
b=-5
Sn= 1/2n(2a+(n-1)b)
-36=1/2n(2*56+(n-1)-5)j
-36=n/2(112+-5n+5)
dikali 2
-72=n(117+-5n)
-72=117n+-5n2
117n+5n2-72=0
(5n+3) (n-24)=0
n= 24 ( diambil angka yang positif)