m²+3m+1=0
m²+1=-3m
m+1/m=-3
(m+1/m)²=（-3）²
m²+2+1/m²=9
m²+1/m²=7
3m²/(m的4次方-m²+1)
=3/(m²-1+1/m²)
=3/(7-1)
=1/2