21.(2023•福建模拟)已知函数f(x)=(x+a)exf(x)=(x+a)e^{x}f(x)=(x+a)ex,a∈Ra\in Ra∈R.(1)讨论f(x)f(x)f(x)在(0,+∞)(0,+\infty )(0,+∞)的单调性;(2)是否存在aaa,x0x_{0}x0,x1x_{1}x1,且x0≠x1x_{0}\ne x_{1}x0=x1,使得曲线y=f(x)y=f(x)y=f(x)在x=x0x=x_{0}x=x0和x=x1x=x_{1}x=x1处有相同的切线?证明你的结论.