公分母4m²-n²;
m/2m+n=m(2m-n)/(4m²-n²)=(2m²-mn)/(4m²-n²),
2n/2n-4m=n/(n-2m)=(-n)(2m+n)/(4m²-n²)=(-2mn-n²)/(4m²-n²),
1/4m^2-n^2;
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