b1 = - 8
q = 1/2
b7 = b1*q^6 = - 8/64 = - 1/8
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b1 = 128
q = 1/4
b4 = b1*q^3 = 128/64 = 2
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b6 = b1*q^5 = 1/27
b1*(1/3)^5 = 1/27
b1 = 243/27
b1 = 9
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b5 = b1*q^4 = 11
b7 = b1*q^6 = 99
q^4/q^6 = 1/9
1/q^2 = 1/9
q^2 = 9
q = 3; q = - 3