x(5y-1)=3yx=3y/(5y-1)3x+4y=9y/(5y-1)+4y=9/5*(5y-1+1)/(5y-1)+4y=13/5+9/5(5y-1)+4(5y-1)/5>=13/5+2*√[9/5(5y-1)*4(5y-1)/5]=13/5+12/5=5当且仅当9/5(5y-1)=4(5y-1)/5,y=-1/10时,等号成立