What is 8 to the 5/8?
The exponent is 5/8 MG not 5/6.
$${{\mathtt{8}}}^{\left({\frac{{\mathtt{5}}}{{\mathtt{8}}}}\right)} = {\mathtt{3.668\: \!016\: \!172\: \!818\: \!684\: \!9}}$$
$${{\mathtt{8}}}^{\left({\frac{{\mathtt{5}}}{{\mathtt{6}}}}\right)} = {\mathtt{5.656\: \!854\: \!249\: \!492\: \!381}}$$