用韦达定理算

题目

用韦达定理算
方程x^2+4x-a=0的一根是另一根的2倍,求a的值,并求出两根.

答案

assume
x1=2x2
since
x1+x2=-4
we get
3x2=-4
that is
x2=-4/3
x1=-8/3
and since
x1*x2=a
we obtain the result
a=32/9