已知函数f(x)=x^2+4x,x大于等于0.4x－x^2,xf(a),求a

已知函数f(x)=x^2+4x,x大于等于0.4x－x^2,xf(a),求a

令x=5
则0.4x-x^2=2-5^2
设a=5
则f(a)=a^2+4a
f(-23)>a^2+4a
437>a^2+4a
a^2+4a-437