若a*a+3a-1=0,则a*a*a+5a*a+5a+18=?可将平方立方写成简写

∵a²+3a-1=0
∴a²+3a=1
a³+5a²+5a+18
=(a³+3a²)+(2a²+6a)-a+18
=a(a²+3a)+2(a²+3a)-a+18
=a×1+2×1-a+18
=a+2-a+18
=20

20，用代入法降幂

即a²+3a=1
原式=a³+3a²+2a²+5a+18
=a(a²+3a)+2a²+5a+18
=a×1+2a²+5a+18
=2a²+6a+18
=2(a²+3a)+18
=2×1+18
=20