∴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.