6.
设\(f(x)=ax-\ln (1+x^{2})\),
\((1)\)当\(a= \dfrac {4}{5}\)时,求\(f(x)\)在\((0,+∞)\)的极值;
\((2)\)证明:当\(x > 0\)时,\(\ln (1+x^{2}) < x\);
\((3)\)证明:\((1+ \dfrac {1}{2^{4}})(1+ \dfrac {1}{3^{4}})…(1+ \dfrac {1}{n^{4}}) < e(n∈N^{*},n\geqslant 2,e\)为自然对数的底数\()\)