设$△ABC$，$P_0$是边$AB$上一定点，满足$P_{0}B=\frac{1}{4}AB$，且对于边$AB$上任一点$P$，恒有$\stackrel{\rightharpoonup }{PB}·\stackrel{\rightharpoonup }{PC}⩾\stackrel{\rightharpoonup }{P_{0}B}·\stackrel{\rightharpoonup }{P_{0}C}$

$\angle ABC=90°$ $\angle BAC=90°$ $AB=AC$ $AC=BC$