17．若定义在RRR上的函数f(x)f(x)f(x)满足：对任意的x1x_{1}x1，x2∈Rx_{2}\in Rx2∈R，都有f(x1+x2)=f(x1)+f(x2)f(x_{1}+x_{2})=f(x_{1})+f(x_{2})f(x1+x2)=f(x1)+f(x2)，且当x>0x>0x>0时，f(x)<0f(x)<0f(x)<0，则((( )))