已知四棱锥\(P-ABCD\)中,\(PA⊥\)平面\(ABCD\),底面\(ABCD\)是边长为\(a\)的正方形,\(PA=b\),\(E\)为\(PD\)中点,\(F\)为\(PA\)上一点,且\(AF= \dfrac {1}{3}b\).
\((1)\)求证:\(CE/\!/\)平面\(BFD\);
\((2)\)设\(AC\)与\(BD\)交于点\(O\),\(M\)为\(OC\)的中点,若点\(M\)到平面\(POD\)的距离为\( \dfrac {1}{5}b\),求\(a\):\(b\)的值.