bn = b1 * q^(n-1) n-й член прогрессии
Sn = b1*(q^n - 1)/(q - 1) сумма n членов
5
b4 - b2 = 24
b2 + b3 = 6
сложим
b4 - b2 + b3 + b2 = 24 + 6
b3 + b4 = 30
b2 + b3 = 6
b1*q^2 + b1*q^3 = q*(b1*q + b1*q^2) = 30 (1)
b1*q + b1*q^2 = 6 (2)
делим (1) на (2)
q*(b1*q + b1*q^2)/(b1*q + b1*q^2) = q = 5
5*b1 + 25*b1 = 6 подставляем в (2)
30 b1 = 6
b1 = 1/5
b4 = b1*q^3 = 1/5 * (5^3) = 25
6
S3 = 39 b1 = 27
S3 = b1*(q^3 - 1)/(q - 1) = b1*(q - 1)(q^2 + q + 1)/(q-1) = b1(q^2 + q + 1)
39 = 27(q^2 + q + 1)
27q^2 + 27q - 12 = 0
9q^2 + 9q - 4 = 0
D = 9^2 - 4*(-4)*9 = 81 + 144 = 225 = 15^2
b12=(-9+-15)/18 = 1/3 -4/3
b1=1/3
b2 = -4/3
2 1/3=7/3
5*7/3-9=35/3-9=(35-27)/3=8/3=2 2/3
Ответ -67 решение -18+15-64=(-18)+15-64=-67
<span>1) 0.8*x+0.3*у=25.56 </span>
<span>у-х=4,9, </span>
<span>2) у=4,9+х </span>
<span>0,8*х+0,3*(4,9+Х)=25,56, отсюда находим х. </span>
<span>1,1*х=24,09, </span>
<span>х= 21,9, </span>
<span>у=26,8 </span>
<span>ответ: сыр стоит 26,8 </span>