2021 AMC 10A Fall Problem 20

Below is the professionally curated solution for Problem 20 of the 2021 AMC 10A Fall, 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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Concepts:quadraticinequalitybounding to limit cases

Difficulty rating: 1820

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

Solution:

A quadratic fails to have two distinct real solutions exactly when its discriminant is nonpositive. Thus we need b24c0andc24b0,b^2-4c\le0\quad\text{and}\quad c^2-4b\le0, or b24cb^2\le4c and c24bc^2\le4b.

From b24cb^2\le4c, we get b416c2b^4\le16c^2. Combining this with c24bc^2\le4b gives b464bb^4\le64b, so b4b\le4.

Now check b=1,2,3,4b=1,2,3,4. The inequalities give respectively (c=1,2),(c=1,2),(c=3),(c=4).(c=1,2),\quad (c=1,2),\quad (c=3),\quad (c=4).

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

Thus, B is the correct answer.

Problem 20 in Other Years