2021 AMC 12A Fall 第 17 题

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17.

有多少个正整数有序对 (b,c)(b, c),使得 x2+bx+c=0x^2 + bx + c = 0x2+cx+b=0x^2 + cx + b = 0 都没有两个不同的实数解?

For how many ordered pairs (b,c)(b, c) of positive integers does neither x2+bx+c=0x^2 + bx + c = 0 nor x2+cx+b=0x^2 + cx + b = 0 have two distinct real solutions?

44

66

88

1212

1616

答案:B
知识点:二次方程不等式
难度评级:1910
解答:

两个二次方程都没有两个不同实根,当且仅当两个判别式都非正: b24cb^2 \le 4cc24bc^2 \le 4b

相乘得 cb2/4c\ge b^2/4,所以 c2bc\le2\sqrt b,迫使取值很小。检查可得: b=1b = 1c{1,2}c \in \{1,2\}b=2b = 2c{1,2}c \in \{1,2\}b=3b = 3c=3c = 3b=4b = 4c=4c = 4;而 b3/28b^{3/2}\le8 时无解。 b4b\le4

这些为 (1,1)(1,1)(1,2)(1,2)(2,1)(2,1)(2,2)(2,2)(3,3)(3,3)(4,4)(4,4),共 66 个有序对。

所以正确答案是 B

Neither quadratic has two distinct real roots exactly when both discriminants are nonpositive: b24cb^2 \le 4c and c24b.c^2 \le 4b.

Combining cb2/4c\ge b^2/4 with c2bc\le2\sqrt b gives b3/28,b^{3/2}\le8, so b4.b\le4. Checking: b=1b = 1 gives c{1,2};c \in \{1,2\}; b=2b = 2 gives c{1,2};c \in \{1,2\}; b=3b = 3 gives c=3;c = 3; and b=4b = 4 gives c=4.c = 4.

That is (1,1),(1,1), (1,2),(1,2), (2,1),(2,1), (2,2),(2,2), (3,3),(3,3), (4,4)(4,4)66 ordered pairs.

Thus, the correct answer is B.

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