15.下列关于函数f(x)=loga(x2−2ax+a2−l)(a>0f(x)=\log _{a}(x^{2}-2ax+a^{2}-l)(a>0f(x)=loga(x2−2ax+a2−l)(a>0,且a≠1)a\ne 1)a=1)说法正确的是((( )))