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CONSUMERS' SURPLUS Suppose that the consumers' demand function for a certain commodity is \(D(q)=50-3 q-q^{2}\) dollars per unit. a. Find the number of units that will be bought if the market price is \(\$ 32\) per unit. b. Compute the consumer willingness to spend to get the number of units in part (a). c. Compute the consumers' surplus when the market price is \(\$ 32\) per unit. d. Use the graphing utility of your calculator to graph the demand curve. Interpret the consumer willingness to spend and the consumers' surplus as areas in relation to this curve.

Short Answer

Expert verified
The number of units bought is 3. Consumers' willingness to spend is 32 dollars per unit. The consumers' surplus is 27 dollars.

Step by step solution

01

Find the number of units bought (Part a)

Given the demand function: \[D(q) = 50 - 3q - q^2\]Set the demand function equal to the market price \(32\): \[50 - 3q - q^2 = 32\].Solve this quadratic equation using the quadratic formula: \[q = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}\] where \(a = -1\), \(b = -3\), and \(c = 18\) (since 50 - 32 = 18).
02

Solve the quadratic equation

Substitute the values into the quadratic formula: \[q = \frac{-(-3) \pm \sqrt{(-3)^2 - 4(-1)(18)}}{2(-1)} = \frac{3 \pm \sqrt{9 + 72}}{-2} = \frac{3 \pm \sqrt{81}}{-2} = \frac{3 \pm 9}{-2}\] This results in two possible solutions: \[q = \frac{3 + 9}{-2} = \frac{12}{-2} = -6\]\[q = \frac{3 - 9}{-2} = \frac{-6}{-2} = 3\]Since the quantity of items cannot be negative, the number of units bought is \(q = 3\).
03

Compute willingness to spend (Part b)

Substitute \(q = 3\) back into the demand function to find the total amount consumers are willing to spend: \[D(3) = 50 - 3(3) - 3^2 = 50 - 9 - 9 = 32\] Thus, the price each consumer is willing to pay for 3 units is 32 dollars per unit.
04

Compute consumers' surplus (Part c)

Consumers' surplus is the area between the demand curve and the price level up to the quantity demanded. It can be calculated using the integral: \[ \text{Surplus} = \frac{1}{2} \times \text{base} \times \text{height} = \frac{1}{2} \times q \times (D(0) - P) \] For \(q = 3\), \[ \text{base} = 3\] and \[ \text{height} = 50 - 32 = 18 \]\[ \text{Surplus} = \frac{1}{2} \times 3 \times 18 = 27 \text{ dollars} \]
05

Graph demand curve and interpret (Part d)

Graph the demand curve \(D(q) = 50 - 3q - q^2\) using a graphing utility. Determine the areas representing consumers' willingness to spend (the area under the demand curve up to \(q = 3\)) and the consumers' surplus (the area between the demand curve and the horizontal line at the market price \(32\), up to \(q = 3\)).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Demand Function
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Market Price
Market price is the current price at which a good or service can be bought or sold. In this exercise, the market price is provided as:
  • \( \$32 \) per unit
To find out how many units of the commodity will be bought at this market price, we need to equate the demand function to the market price and solve for the quantity demanded:
\( D(q) = 32 \)
  • \( 50 - 3q - q^2 = 32 \)
By solving this equation, students can determine the quantity demanded at this price.
Quadratic Equation
A quadratic equation is a polynomial equation of the form \( ax^2 + bx + c = 0 \). In our situation, the quadratic equation arises when we set the demand function equal to the market price:
  • \( 50 - 3q - q^2 = 32 \)
Rearranging, we get:
  • \( -q^2 - 3q + 18 = 0 \)
This standard form allows us to solve for \( q \) using the quadratic formula:
  • \( q = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a} \)
Here, \( a = -1 \), \( b = -3 \), and \( c = 18 \).
By solving, we find the possible values for \( q \) and determine the valid (non-negative) solution.
Willingness to Spend
Willingness to spend refers to the total amount of money consumers are ready to pay to purchase a specific number of units. It can be found by integrating the demand function from 0 to the quantity demanded. In this case, the quantity demanded when the market price is \( 32 \) is found to be 3 units. Let's integrate to find the willingness to spend:
  • \( \int_{0}^{3} (50 - 3q - q^2) dq \)
By solving, we get the total area under the demand curve up to \( q = 3 \), representing the total willingness to spend.
Integral
An integral calculates the area under a curve in a given interval. This is particularly useful in finding consumers' surplus. Here, we calculate the area under the demand curve from 0 to the quantity demanded:
  • \( \int_{0}^{3} (50 - 3q - q^2) dq \)
After solving the integral, we subtract the area that consumers actually pay, given by the product of the market price and the quantity demanded. This gives us the consumers' surplus:
  • \( \text{Surplus} = \int_{0}^{3} (50 - 3q - q^2) dq - (32 \times 3) \)
By working out these calculations, we get to understand the additional benefit consumers receive from purchasing at a market price lower than they are willing to pay.

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