1/x+1/y
=(1/x+1/y)*5*1/5
=(1/x+1/y)*(x+4y)*1/5
=(1+4+4y/x+x/y)*1/5
≥(5+2√4)*1/5
=9/5
∴1/x+1/y最小值为9/5
祝你学业进步!