已知a,b是方程x2-x-1=0的两个实数根,求a3+2b-5的值

由题意得：a^2-a-1=0
则a^2=a+1
由未达定理得：a+b=1
则原式=a^2*a+2b-5
=(a+1)*a+2b-5
=a^2+a+2b-5
=a+1+a+2b-5
=2a+2b-4
=2(a+b)-5
=2-5
=-3
在我回答的右上角点击【采纳答案】 那2a^2+5b^2的值为多少由题意得：a^2-a-1=0,b^2-b-1=0
则a^2=a+1,b^2=b+1
则2a^2+5b^2=2(a+1)+5(b+1)
=2a+1+5b+5
=2a+5b+6
=2(a+b)+3b+6
=2+3b+6
=8+3b