Question

daca se cunosc doi termeni ai unei progresii aritmetice (c indice n):c6=50 si c₂₀= -20,sa se gaseasca c19, c₇ si c25

Answer 1

c6 = c1 + 5r = 50

c20 = c1 + 19r = -20

Scadem a doua relatie din prima:

c1+5r-(c1+19r) = 50-(-20)

c1+5r-c1-19r = 50+20

-14r = 70

r = -70/14, adica r = -5

Dar c1+5r = 50, adica c1+5*(-5) = 50, adica c1-25 = 50, deci c1 = 75

Atunci c19 = c1+18r = 75+18*(-5) = 75-90 = -15

c7 = c1+6r = 75+6*(-5) = 75-30 = 45

c25 = c1+24r = 75+24*(-5) = 75-120 = -45