5.
设函数\(f(x)\)的定义域是\(R\),对于任意实数\(m\),\(n\),恒有\(f(m+n)=f(m)⋅f(n)\),且当\(x > 0\)时,\(0 < f(x) < 1\).
\((1)\)求证:\(f(0)=1\),且当\(x < 0\)时,有\(f(x) > 1\);
\((2)\)判断\(f(x)\)在\(R\)上的单调性;
\((3)\)设集合\(A=\{(x,y)|f(x^{2})⋅f(y^{2}) > f(1)\}\),\(B=\{(x,y)|f(ax-y+2)=1\),\(a∈R\}\),若\(A∩B=\varnothing \),求\(a\)的取值范围.