7

Set 8: Linear Inequalities (Intermediate)

Explanation

Answer: A

Which system of inequalities best describes the shaded region inside a triangle with vertices at (0,0)(0,0), (4,0)(4,0), and (0,4)(0,4)?

A.

x0,y0,yx+4x \geq 0, y \geq 0, y \leq -x + 4

✓ Correct
B.

x0,y0,yx+4x \geq 0, y \geq 0, y \geq -x + 4

C.

x4,y4,yx+4x \leq 4, y \leq 4, y \leq -x + 4

D.

x>0,y>0,y<x+4x > 0, y > 0, y < -x + 4

Detailed Explanation

Choice A is correct. Choice A is the correct answer. Define the three boundary lines. 1. Bottom edge: The x-axis (y=0y=0). Shading is above, so y0y \geq 0. 2. Left edge: The y-axis (x=0x=0). Shading is right, so x0x \geq 0. 3. Hypotenuse: Line connecting (4,0)(4,0) and (0,4)(0,4). Slope is -1, y-intercept is 4. Equation: y=x+4y = -x + 4. Shading is below, so yx+4y \leq -x + 4. 4. Combine: x0,y0,yx+4x \geq 0, y \geq 0, y \leq -x + 4 Strategic Tip: For triangles in the first quadrant, you usually need x0x \geq 0 and y0y \geq 0 plus one diagonal line. Choice B is incorrect because it shades above the hypotenuse. Choice C is incorrect because x4x \leq 4 and y4y \leq 4 create a square, not a triangle. Choice D is incorrect because it uses strict inequalities (dashed lines).

Key Steps:

The correct answer is x0,y0,yx+4x \geq 0, y \geq 0, y \leq -x + 4

Why others are wrong:
B: Choice B is incorrect and may result from a calculation error.
C: Choice C is incorrect and may result from a calculation error.
D: Choice D is incorrect and may result from a calculation error.

🎯 Keep Practicing!

Master all sections for your best SAT score