因为a+1/a=3,a^2+1/a^2=7,a^3+1/a^3=18 所以(a^2+1/a^2)(a^3+1/a^3)=7*18 即a^5+a^2/a^3+a^3/a^2+1/a^5=126 a^5+1/a+a+1/a^5=126 a^5+3+1/a^5=126 所以a^5+1/a^5=123
