Question

Prosze o pomoc ! Oblicz miejsca zerowe ciągow :

an = log(n-7) bn= n3 - 5n2 - 4n +20

Answer 1

an = log (n - 7)

n∈N   ∧   n ≥ 1

 

0 = log (n - 7)

log 1 = log (n - 7)

1 = n - 7

n = 8

 

bn= n³ - 5n² - 4n + 20

n∈N   ∧   n ≥ 1

 

0 = n³ - 5n² - 4n + 20

0 = n²(n-5) - 4(n-5)

0 = (n² - 4)(n-5)

0 = (n-2)(n+2)(n-5)

n - 2 = 0   v    n + 2 = 0   v    n - 5 = 0

n = 2         v    n = -2 ∉ N    v   n = 5

n = 2   v   n = 5