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A biologist took a count of the number of migrating waterfowl at a particular lake, and recounted the lakes population of waterfowl on each of the next six weeks. Find a quadratic function that models the data as a function of x, the number of weeks. Use the model to estimate the number of waterfowl at the lake on week 8.

week 0 1 2 3 4 5 6

population 635 644 719 860 1,067 1,340 1,769

Verified Answer :

Quadratic function ( in standard form ):
y = a x² + b x + c
x = 0:
635 = 0 + 0 + c
c = 635
x = 1:
644 = a + b + 635
a + b = 9
x = 2:
719 = 4 a + 2 b + 635
4 a + 2 b = 84
and now we have a system:
a + b = 9   /   ( - 2 )
4 a + 2 b = 84
-------------------------
- 2 a + 2 b = - 18
+
  4 a + 2 b  = 84
-------------------------
   2 a = 66
  a = 33;   b = - 24;
  Function is:
  f ( x ) = 33 x² - 24 x + 635
 f ( 8 ) = 33
64 - 24 * 8 + 635 = 2,112 - 192 + 635 = 2,555
  Answer: The number of waterfowl on week 8 would be 2,555. 

The number of waterfowl on 8 week would be \(\boxed{2555}\).

Further explanation:

The general form of the quadratic equation is given by,

\(\boxed{a{x^2} + bx + c}\)

Here, a is the coefficient of \({x^2}\), b is the coefficient of \(x\) and \(c\) is the constant term.

Given:

The population of the lake in 0, 1, 2, 3, 4, 5 and 6 week is 635, 644, 719, 860, 1067, 1340 and 1769 respectively.

Calculation:

Consider \(x\) denote the number of weeks.

Quadratic function that models the data as the function of \(x\) can be expressed as,

\(\boxed{N=a{x^2}+bx +c}\)

Here, \(N\) denotes the population in \({n^{th}}\) week.

The population of lake in the beginning is 635. Therefore, value of \(x\) is zero and value of \(N\) is 635.

Substitute \(0\) for \(x\) and \(635\) for \(N\) in quadratic equation \(N = a{x^2} + bx + c\).

\(\begin{aligned} 635&=a{\left(0\right)^2}+b\left( 0 \right)+c\\ 635&=0+0+c\\ 635&=c\\\end{aligned}\)

The population of lake after one week is \(644\). Therefore, value of \(x\) is one and value of \(N\) is \(644\).

Substitute \(1\) for \(x\) and \(644\) for \(N\) in quadratic equation \(N = a{x^2} + bx + c\).

\(\begin{aligned} 644 &= a{\left( 1 \right)^2} + b\left( 1 \right) + c \\ 644 &= a + b + c \\ 644 &= a + b + 635 \\ 644 - 635 &= a + b \\ 9 &= a + b \\\end{aligned}\)

The population of lake after two week is \(719\). Therefore, value of \(x\) is two and value of \(N\) is \(719\).

Substitute 2 for \(x\) and 719 for \(N\) in quadratic equation \(N = a{x^2} + bx + c\).

\(\begin{aligned} 719 &= a{\left( 2 \right)^2} + b\left( 2 \right) + c \\ 719 &= 4a + 2b + 635 \\ 719 - 635 &= 4a + 2b \\ 84 &= 2\left( {2a + b} \right) \\ \frac{{84}}{2} &= \left( {2a + b} \right) \\ 42 &= \left( {2a + b} \right) \\ \end{aligned}\)

Solve the equation \(a + b = 9\) to obtain the value \(a\).

\(a = 9 - b\)

Substitute \(9 - b\) for in equation \(42 = 2a + b\).

\(\begin{aligned} 42 &= 2\left( {9 - b} \right) + b \\42 &= 18 - 2b + b \\ 42 - 18 &=- b \\24 &=- b\\- 24 &= b \\ \end{aligned}\)

Substitute \(-24\) for \(b\) in equation \(a = 9 - b\) to obtain the value of a.

\(\begin{aligned}a&= 9-\left( {-24} \right) \\&= 9+24\\&= 33 \\\end{aligned}\)

To obtain the quadratic function substitute 33 for a, -24 for b and 635 for c in equation \(N = a{x^2} + bx + c\).

\(N = 33{x^2} - 24x + 635\)

Substitute 8 for \(x\) in above equation to obtain the number of waterfowl at the lake after week \(8\).

\(\begin{aligned} N &= 33{\left( 8 \right)^2} - 24\left( 8 \right) + 635 \\ &=33\left( {64} \right) - 192 + 635 \\ &= 2112 - 192 + 635 \\ &= 2555 \\ \end{aligned}\)

The number of waterfowl on \(8\) week would be \(\boxed{2555}\).

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Answer details:

Grade: Middle School

Subject: Mathematics

Chapter: Quadratic equation

Keywords: quadratic equation, polynomial, square root, real numbers, coefficients, constant term, general form of quadratic equation, waterfowl, lake, population, migrating, a biologist, model, estimate, week, recounted, function.