f(x)=x^2+2x=(x+1)^2-1(x>=0),
开口向上,对称轴x=-1
故f(x)在[0,+∞)是增函数
f(x)=2x-x^2=-(x-1)^2+1(xf(a)
故2-a^2>a
解得 -2