7.下列函数中,既是偶函数,又满足对任意的x1x_{1}x1,x2∈(0,+∞)x_{2}\in (0,+\infty )x2∈(0,+∞),当x1>x2x_{1}>x_{2}x1>x2时,都有f(x1)>f(x2)f(x_{1})>f(x_{2})f(x1)>f(x2)的是((( )))