[2/(m+n)³(1/m+1/n)+1/(m²+2mn+n²)(1/m²+1/n²)]÷[(m-n)/(m³n³)]
={2/[(m+n)²mn]+(m²+n²)/[(m+n)²m²n²]}x[(m³n³)/(m-n)]
=(2mn+m²+n²)/[(m+n)²m²n²]x[(m³n³)/(m-n)]
=(mn)/(m-n)