若a是方程x²+3x-1=0的根,求a²+1/a²
问题描述:
若a是方程x²+3x-1=0的根,求a²+1/a²
答
a²+1/a²=(a^4+1)/a²
=(a²(a²+3a-1)-3a^3+a²+1)/a²
=(a²(a²+3a-1)-3a(a²+3a-1)+9a²-3a+a²+1)/a²
=(a²(a²+3a-1)-3a(a²+3a-1)+11a²-(a²+3a-1))/a²
=(0-0+11a²-0)/a²
=11