Odpowiedzi

  • Użytkownik Zadane
2013-07-17T09:22:45+02:00

a1 = 12

a2 = 6

S20 = ?

 

n = 20

r = a2 - a1 = 6 - 12 = -6

 

an = a1 + (n - 1) * r

a20 = 12 + (20 - 1) * (-6)

a20 = 12 + 19 * (-6)

a20 = 12 - 114

a20 = -102

 

S20 = [ (a1 + a20) / 2 ] * n

S20 = [ (12 + (-102)) / 2 ] * 20

S20 = [(-90) / 2 ] * 20

S20 = -45 * 20

S20 = - 900   -----------  odpowiedz