2014 AMC 10A Problem 18

Attempt Problem 18 of the 2014 AMC 10A 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 2014 AMC 10A solutions, or check the answer key.

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18.

A square in the coordinate plane has vertices whose yy-coordinates are 0,0, 1,1, 4,4, and 5.5. What is the area of the square?

1616

1717

2525

2626

2727

Answer: B
Concepts:coordinate geometrysquare (geometry)vector
Difficulty rating: 1720
Solution:

Opposite vertices of a square have the same average yy-coordinate. Thus the opposite pairs must have yy-coordinates (0,5)(0,5) and (1,4)(1,4), the two pairs with equal sum. In particular, a vertex with yy-coordinate 00 is adjacent to vertices with yy-coordinates 11 and 44.

Let one vertex be A=(0,0)A=(0,0), and let its adjacent vertex with yy-coordinate 11 be B=(x,1)B=(x,1) with x>0x>0.

Rotating the side vector (x,1)(x,1) by 9090^\circ gives the next side vector (1,x)(-1,x), so another vertex has y-coordinate xx. The remaining y-coordinates are 44 and 55, hence x=4x=4.

The side length squared is AB2=x2+12=42+1=17AB^2=x^2+1^2=4^2+1=17, which is the area of the square.

Thus, B is the correct answer.

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