函数是偶函数，f(x)=f(-x)x≤0时，-x≥0f(-x)=f(x)=e^(-x) -xf(x)=e^x +xf(1)=e+1f'(x)=e^x +1f'(1)=e+1y-(e+1)=(e+1)(x-1)切线方程为：(e+1)x-y=0
