f(x)=-1/2x²+x+4
区间[m,n],值域为[2m,2n]
f(m)=(-1/2)m²+m+4=2m,(m+4)(m-2)=0,m1=-4,m2=2
f(n)=(-1/2)n²+n+4=2n,n1=-4,n2=2.
m