a²+3a+1=0 两边同时除以a
a+3+1/a=0
a+1/a=-3
a²+1/a²
=(a+1/a)²-2
=（-3)²-2
=7
a的4次方+1/a的4次方
=(a²+1/a²)²-2
=7²-2
=47