B3=0.05,b5=0.45 найдите S8

Question

B3=0.05,b5=0.45 найдите S8

Answer 1

B5/ b3 = q^2
q^2=0.45/0.05 = 9 
q = 3
S8 = b1(q^8 - 1)/ (q - 1)
b3 = b1 * q^2
b1 = 0.05 / 3^2 = 5/100 * 1/9  = 5/900 = 1/180
S8 = 1/180 * (3^8 - 1)/ (3-1)
S8 = 1/180 * (6561 - 1)/ 2
S8 = 1/180 * 3280 = 3280 / 180 = 1640 / 90= 820 / 45 = 164 / 9

Comments

спасибо большое