2019 AMC 8 Problem 4

Attempt Problem 4 of the 2019 AMC 8 below, then check your answer against the video solution and professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2019 AMC 8 solutions, or check the answer key.

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

4.

Quadrilateral ABCDABCD is a rhombus with perimeter 5252 meters. The length of diagonal AC‾\overline{AC} is 2424 meters. What is the area in square meters of rhombus ABCD?ABCD?
6060
9090
105105
120120
144144
Answer: D
Concepts:rhombusPythagorean Theorem
Small hint:
Each rhombus side is 52÷452\div4
Big hint:
The diagonals of a rhombus are perpendicular and bisect each other
Video solution:
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Written solution:
We can split the rhombus up into 44 triangles, each of which looks like this.
We know that this is a right triangle, since the diagonals of a rhombus are perpendicular, and we know the hypotenuse is 52÷4=13.52 \div 4 = 13.
Using the Pythagorean theorem, we get the other leg to be 132−122=25=5. \sqrt{13^2 - 12^2} = \sqrt{25} = 5.
This means that the area of the rhombus is 4⋅5⋅122=4⋅30=120. 4 \cdot \dfrac{5 \cdot 12}{2} = 4 \cdot 30 = 120. Thus, the correct answer is D.
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