p=x/10,q=2√x/5,
设乙投入x万元,甲投入9-x万元
获利f(x)=p+q=(9-x)/10+2√x/5
=-x/10+0.4√x+0.9
=(-x+4√x-4)/10+1.3
=1.3-(√x-2)^2/10