Question

progresie geometrica ​

Answer 1

Răspuns:

Explicație pas cu pas:

b4 = b1*q^3

b1*q^3 - 27b1 = -57

b1*q^2 + 3b1*q + 9b1 = 57

b1*(q^3 - 27) = -57

b1*(q^2 +3q + 9) = 57

(q^3 - 27)/(q^2 + 3q + 9) = -1

q^3 - 27 = -(q^2 + 3q + 9)

q^3 + q^2 + 3q - 27 +9 = 0

q^3 + q^2 + 3q - 18 = 0

q = 2

b1 = -57/(8 - 27) = -57/-19 = 3

b2 = 3*2 = 6

b3 = 6*2 = 12

b4 = 12*2 = 24

b5 = 24*2 = 48

S5 = 3 + 6 + 12 + 24 + 48 = 93