2022 AMC 12A Problem 5

Attempt Problem 5 of the 2022 AMC 12A 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 2022 AMC 12A solutions, or check the answer key.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

Let the taxicab distance between points $(x_1,y_1)$ and $(x_2,y_2)$ in the coordinate plane be given by $|x_1-x_2|+|y_1-y_2|.$ For how many points $P$ with integer coordinates is the taxicab distance between $P$ and the origin less than or equal to $20?$

$441$

$761$

$841$

$921$

$924$

Answer: C

Concepts:lattice point arithmetic sequence

Difficulty rating: 1350

Solution:

For each $k\ge1,$ the set $|x|+|y|=k$ contains exactly $4k$ lattice points, and $k=0$ gives the single origin.

The total is $\begin{aligned} &1+\sum_{k=1}^{20}4k=1+4\cdot\frac{20\cdot21}{2} \\ &=1+840=841. \end{aligned}$

Thus, the correct answer is C.

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