2x+4y=1则有x=0.5-2y,则x^2=0.25+4y^2-2y
所以x^2+y^2=0.25+4y^2-2y+y^2=5y^2-2y+0.25=5(y^2-0.4y)+0.25=5[(y-0.2)^2-0.04]+0.25
则当y=0.2时x^2+y^2取最小值=5*(-0.04)+0.25=0.05=1/20
D