a是方程y^2-y-1=0的一个根
则a²-a-1=0
则a²-a=1
a^3-2a+3
=a³-a²+a²-2a+3
=a(a²-a)+a²-2a+3
=a×1+a²-2a+3
=a²-a+3
=1+3
=4