q = b2/b1 = -0.5
bn = b1*q^(n-1)
b4 = b1 * q ^ 3
b4 = 64 * (-0.5)^3 = -8
b1*q=-32
64*q=-32
q=-1/2
b4=b1*q^3 = 64*(-1/2)^3= 64*(-1/8) = -8
