Mathematics Grade 9 15 min

Minimum and maximum area and volume

Minimum and maximum area and volume

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1

Introduction & Learning Objectives

Learning Objectives Model a real-world problem involving area with a quadratic function. Determine the dimensions that maximize the area of a rectangle for a fixed perimeter. Determine the dimensions that minimize the surface area of a square-based prism for a fixed volume. Use the vertex of a parabola to find the maximum or minimum value in an optimization problem. Interpret the meaning of the vertex's coordinates in the context of area and volume problems. Solve problems where one side of a rectangular area is enclosed by a natural barrier. Ever wondered how to build the biggest possible garden with a limited amount of fence? 🧑‍🌾 Let's use math to find out! This tutorial will teach you how to use quadratic functions to solve optimization problems. You'll...
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Key Concepts & Vocabulary

TermDefinitionExample OptimizationThe process of finding the best possible value (the maximum or minimum) for a given situation, usually under specific limitations or constraints.Finding the dimensions of a rectangular pen that give the maximum possible area for 100 meters of fencing. ConstraintA limitation or condition that must be satisfied in a problem. This is often a fixed value, like a set perimeter or volume.You have exactly 40 meters of fencing to build a garden. The perimeter is the constraint. Quadratic FunctionA function that can be written in the form f(x) = ax² + bx + c, where 'a' is not zero. Its graph is a parabola.The area of a rectangle with a fixed perimeter can often be modeled by a quadratic function, like A(w) = -w² + 20w. ParabolaA U-shaped curve that is th...
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Core Formulas

Vertex Formula (x-coordinate) x = -b / (2a) For any quadratic function in the form f(x) = ax² + bx + c, this formula finds the x-coordinate of the vertex. This tells you the value of the input (like width or length) that results in the maximum or minimum output (like area or volume). Optimal Rectangle (Fixed Perimeter) A square maximizes area. For any given perimeter of a four-sided rectangle, a square (where length equals width) will always provide the maximum possible area. Optimal Square-Based Prism (Fixed Volume) A cube minimizes surface area. For any given volume of a square-based prism, a cube (where length, width, and height are equal) will always have the minimum possible surface area.

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Sample Practice Questions

Challenging
A rectangular field is divided into two equal adjacent pens by a fence parallel to the width. If 300 meters of fencing are used in total, what is the maximum possible area of the entire field (both pens combined)?
A.3750 m²
B.5625 m²
C.7500 m²
D.2500 m²
Challenging
An open-topped square-based box must have a volume of 500 cm³. What is the side length of the base (to one decimal place) that will minimize the amount of material used?
A.7.9 cm
B.10.0 cm
C.5.0 cm
D.8.7 cm
Challenging
A rectangular garden with an area of 800 m² is to be fenced. The fencing for the two sides representing the length costs $12/m, and the fencing for the two sides representing the width costs $6/m. What width (in meters) will minimize the total cost of the fence?
A.20 m
B.28.3 m
C.10 m
D.40 m

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