已知X1,X2为X^2+3X+1=0的两实根,则X1^3+8X2+20等于多少?
问题描述:
已知X1,X2为X^2+3X+1=0的两实根,则X1^3+8X2+20等于多少?
答
x^2+3x+1=0
x^2=-3x-1
所以,x1^2=-3x1-1,
韦达定理得:x1+x2=-3
x1^3+8x2+20
=x1*(-3x1-1)+8x2+20
=-3x1^2-x1+8x2+20
=-3(-3x1-1)-x1+8x2+20
=8x1+3+8x2+20
=8(x1+x2)+23
=8*(-3)+23
=-1