三次方方程求解公式
比如说X3-9X2+25X-42=0
答案是X=6,但是有公式可以解吗?
人气：101 ℃ 时间：2020-03-28 10:01:28
解答
X^3-9X^2+25X-42=0
(x^3-6x^2)-(3x^2-25+42)=0
x^2(x-6)-(x-6)(3x-7)=0
(x-6)(x^2-3x+7)=0
x^2-3x+7>0恒成立
所以x-6=0
x=6
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