∵(x-y)²≥0 ==>x²+y²≥2xy (应用完全平方公式)∴│(x+y)/(x²+y²)│≤│(x+y)/(2xy)│≤(1/│x│+1/│y│)/2∵当x->0和y->0时,(1/│x│+1/│y│)/2->0∴lim[(x+y)/(x²+y²)]=0 (...