S=b1* (1/1-q)
b2=b1q
q=b2/b1=24/(-120)=-1/5
S=(-120)*1/(1+1/5)=(-120)*1/6/5=(-120)*5/6=-20*5=-100
x^3 - 81x = 0
x(x^2 - 81) = 0
x = 0 или (x-9)(x+9)=0
x = 0 или
x - 9 = 0
x = 9 ,
x + 9 = 0
x = - 9