Question

найти b5 и S4 если b2=15 b3=25

Answer 1

b3 = b2*q
q = b3 / b2 = 25 / 15 = 5/3
b5 = b4 * q = b3 * q² = 25 * 25 / 9 = 625 / 9
S4 = b1 * (q^4 - 1) / (q-1)
b2 = b1*q 
b1 = 15 / (5/3) = 15*3 / 5 = 9
S4 = 9*(625/81 - 1) / (5/3 - 1) = 544*3 / (9*2) = 272/3