a是X的平方+3X+1=0的根
则:a²+3a+1=0
a²=-3a-1
a的4次方+a的3次方-6a的平方-5a+5
=(-3a-1)²+a(-3a-1)-6a²-5a+5
=9a²+6a+1-3a²-a-6a²-5a+5
=1+5
=6