∴a? -ab-4b? =0
∵|a|= 1
∴a? -4b? =ab is 1-4b? =|b|cosθ, θ is the included angle between a and B.
∴-|b|≤ 1-4b? ≤|b|
∴4b? -|b|- 1≤0
4b? +|b|- 1≥0
The minimum value of ∴|b| is (-1+√ 17)/8.