2021 AMC 10A Fall Problem 20

Attempt Problem 20 of the 2021 AMC 10A Fall below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2021 AMC 10A Fall solutions, or check the answer key.

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20.

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

1010

1212

Answer: B
Concepts:quadraticinequalitybounding to limit cases
Difficulty rating: 1820
Solution:

A quadratic fails to have two distinct real solutions exactly when its discriminant is nonpositive. Thus we need b24c0c24b0,b^2-4c\le0\qquad c^2-4b\le0, or b24cb^2\le4c and c24b.c^2\le4b.

From b24c,b^2\le4c, we get b416c2.b^4\le16c^2. Combining this with c24bc^2\le4b gives b464b,b^4\le64b, so b4.b\le4.

Now check b=1,2,3,4.b=1,2,3,4. The inequalities give respectively (c=1,2),(c=1,2),(c=3),(c=4). \begin{gathered} (c=1,2),\quad (c=1,2), \\ \quad (c=3),\quad (c=4). \end{gathered}

There are 2+2+1+1=62+2+1+1=6 ordered pairs.

Thus, B is the correct answer.

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