2019 AMC 12B 第 21 题

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所有题目均经美国数学协会(MAA)官方合法授权使用。

21.

有多少个实系数二次多项式满足:根的集合等于系数的集合?(说明:若多项式为 ax2+bx+c, a0ax^2+bx+c,\ a\neq0,根为 rrss,则要求 {a,b,c}={r,s}\{a,b,c\}=\{r,s\}。)

How many quadratic polynomials with real coefficients are there such that the set of roots equals the set of coefficients? (For clarification: If the polynomial is ax2+bx+c, a0,ax^2+bx+c,\ a\neq0, and the roots are rr and s,s, then the requirement is that {a,b,c}={r,s}.\{a,b,c\}=\{r,s\}.)

33

44

55

66

无穷多个

infinitely many

答案:B
知识点:韦达定理分类讨论方程组
难度评级:2220
解答:

如果三个系数都等于同一个值 u,u,多项式就是 u(x2+x+1),u(x^2+x+1),其根不等于 u.u. 因此系数集合和根集合都有两个不同的值,所以恰有两个系数相同。由韦达定理,r+s=bar+s=-\dfrac{b}{a}rs=ca.rs=\dfrac{c}{a}.

先设 a=b=ua=b=uc=v.c=v. 根为 u,v,u,v,所以韦达定理给出 u+v=1u+v=-1uv=v/u.uv=v/u. 因而 v(u21)=0,v(u^2-1)=0,得到 x2+x2x^2+x-2x2x.-x^2-x.

如果 b=c=vb=c=va=u,a=u,同一个乘积方程给出 v(u21)=0,v(u^2-1)=0,而和的方程只留下 u=1, v=12.u=1,\ v=-\tfrac12. 这给出 x212x12.x^2-\tfrac12x-\tfrac12.

最后,如果 a=c=ua=c=ub=v,b=v,乘积方程给出 uv=1,uv=1,所以 v=1/u.v=1/u. 和的方程变为 u3+u+1=0.u^3+u+1=0. 这个严格递增的三次函数有唯一实根 u,u,因此恰好再得到一个多项式 ux2+1ux+u.ux^2+\dfrac1u x+u. 所以共有 44 个多项式。

所以 B 是正确答案。

If all three coefficients had one value u,u, the polynomial would be u(x2+x+1),u(x^2+x+1), whose roots do not equal u.u. Thus the coefficient and root sets both have two distinct values, so exactly two coefficients coincide. By Vieta's formulas, r+s=bar+s=-\dfrac{b}{a} and rs=ca.rs=\dfrac{c}{a}.

First suppose a=b=ua=b=u and c=v.c=v. The roots are u,v,u,v, so Vieta gives u+v=1u+v=-1 and uv=v/u.uv=v/u. Hence v(u21)=0,v(u^2-1)=0, producing x2+x2x^2+x-2 and x2x.-x^2-x.

If b=c=vb=c=v and a=u,a=u, the same product equation gives v(u21)=0,v(u^2-1)=0, while the sum equation leaves only u=1, v=12.u=1,\ v=-\tfrac12. This gives x212x12.x^2-\tfrac12x-\tfrac12.

Finally, if a=c=ua=c=u and b=v,b=v, the product equation gives uv=1,uv=1, so v=1/u.v=1/u. The sum equation becomes u3+u+1=0.u^3+u+1=0. This strictly increasing cubic has one real root u,u, producing exactly one more polynomial, ux2+1ux+u.ux^2+\dfrac1u x+u. Therefore there are 44 polynomials.

Thus, B is the correct answer.

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