a^2=a+1
两边平方
a^4=a^2+2a+1=(a+1)+2a+1=3a+2
a^5=a*a^4=a(3a+2)=3a^2+2a=3(a+1)+2a=5a+2
所以a^5-5a+2=5a+2-5a+2=4