证明：∵m²+n²﹥m²-n²,m²+n²﹥2mn(m²+n²)²=m^4+2m²n²+n^4(m²-n²)²+(2mn)²=m^4+n^4-2m²n²+4m²n²=m^4+n^4+2m²...m^4是啥咋读m^4就是m的四次方。