x的平方+（2m+3）x+m的平方=0的两个不相等的实数根
a+B=-(2m+3),aB=m^2
1/a+1/B
=(a+B)/aB
=-(2m+3)/m^2
=-1
m^2=2m+3
m^2-2m-3=0
(m-3)(m+1)=0
m1=3,m2=-1