Найти пятый член прогресс,в которой b1+b4=36.b1+b2=18.

B1+b4=b1+b1q³=b1(1+q³)=36⇒b1=36/(1+q³)
b1+b2=b1+b1q=b1(1+q)=18⇒b1=18/(1+q)
36/(1+q)(1-q+q²)=18/(1+q)
1-q+q²=2
q²-q-1=0
D=1+4=5
q1=(1-√5)/2⇒b1=18:(1+(1-√5)/2)=36/(3-√5)⇒b5=36/(3-√5)*(1-√5)^4/16=
=36/(3-√5)*4(3-√5)²/16=9(3-√5)
q1=(1+√5)/2⇒b1=18:(1+(1+√5)/2)=36/(3+√5)⇒b5=9(3+√5