3. For an equilateral $\triangle ABC$ with side length $1$, $P$ is a point on side $AB$. $PQ \perp BC, \quad QR \perp AC, \quad RS \perp AB$ (where $Q, R, S$ are the feet of the perpendiculars). If $SP=\frac{1}{4}$, then $AP=$
3. $\frac{1}{9}$ or $\frac{5}{9}$,