(bn);...;1;x;9;-27;
x-?

b1 = 1, b2=x, b3=9, b4=-27
q=b2/b1 = b3/b2=b4/b3=.........,q=-27/9 =-3
b2=b1.q,b2= 1.(-3)=-3
x=b2, x = -3
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