Question

若M+N=5，MN=-3，则1/M²+1/N²=

Answer 1

1/M²+1/N²
=(n²+m²弊洞洞租枯)/m²n²
=[(m+n)²-2mn]/(mn)²颤哪
=(5²+6)/9
=31/9

Answer 2

1/M²+1/没伏没N²枯纳
=(M²+N²)/M²N²
=(M²+N²+2MN-2MN)/M²N²厅氏
=[(M+N)²-2MN]/(-3)²
=[5²-2*(-3)/(-3)²
=(25+6)/9
=31/9