x>0时
y=(3/16)x^2+3/x
=(3/16)x^2+3/2x+3/2x
≥3[(3/16)x^2*3/2x*3/2x]^(1/3)
y≥3*3/4=9/4
所以3/16x^2=3/2x,即x=2时,y有最小值9/4
这题应该有个前提条件x>0