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
