Question

不解方程 求下列 1.x^2-3x-18=0 2.x^2+3x-5=0 3.x^2-4x-7=0 4.3^2-6x-9=

Answer 1

用韦达定理先求出x1+x2=-b/a,x1乘以x2=c/a,

再计算1/x1+1/x2=(x1+x2)/x1x2,

x1^2+x2^2=(x1+x2)^2-2x1x2,

（x1-x2）^2==(x1+x2)^2-4x1x2