如图所示，\(PA\)为圆\(O\)的切线，\(A\)为切点，\(PO\)交圆\(O\)于\(B\)，\(C\)两点，\(PA=20\)，\(PB=10\)，\(∠BAC\)的角平分线与\(BC\)和圆\(O\)分别交于点\(D\)和\(E\)．
\((1)\)求证：\( \dfrac {AB}{AC}= \dfrac {PA}{PC}\)．
\((2)\)求\(AD⋅AE\)的值．