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2008 AMC 12A Problem 14

Problem 14 of 25IntermediateAlgebraGeometry

What is the area of the region defined by the inequality 3x18+2y+73?|3x - 18| + |2y + 7| \le 3?

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Solution

The region is a rhombus centered at (6,72).\left(6, -\tfrac{7}{2}\right). Setting 2y+7=02y + 7 = 0 gives 3x183,|3x - 18| \le 3, so x[5,7],x \in [5, 7], a horizontal diagonal of length 2.2. Setting 3x18=03x - 18 = 0 gives 2y+73,|2y + 7| \le 3, so y[5,2],y \in [-5, -2], a vertical diagonal of length 3.3. The area of the rhombus is half the product of its diagonals, 1223=3. \dfrac{1}{2} \cdot 2 \cdot 3 = 3. Thus, A is the correct answer.

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Concepts: absolute value · rhombus · area

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