x/y=x-y, x=y+x/y≥2√(y*x/y)=2√x；∵x>0∴不等式平方得x^2≥4x ∴x≥4 选C
x/y=x-y, x=xy-y^2, (y-1)x=y^2,由重要不等式： x=y^2/(y-1)=y-1+1/(y+1)+2>=4
x/y=x-y
x=(x-y)y
x=xy-y*y
x-xy=-y*y
x(1-y)=-y*y
x=y*y/(y-1)=((y-1)^2+2(y-1)+1)/(y-1)≥y-1+2+1/(y-1)≥4
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