Big Ideas Math Algebra 2, 2014
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Big Ideas Math Algebra 2, 2014 View details
5. Using Recursive Rules with Sequences
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Exercise 67 Page 450

Practice makes perfect
a Let's analyze the given recursive sequence.

f(1)=3, f(2)=10 f(n)=4+2f(n-1)-f(n-2) We are asked to write the first five terms of f(n). Let's do it!

n f(n-2) f(n-1) 4+2f(n-1)-f(n-2) f(n)=4+2f(n-1)-f(n-2)
1 - - - f(1)=3
2 - - - f(2)=10
3 f(1)= 3 f(2)= 10 4+2( 10)- 3= 21 f(3)= 21
4 f(2)= 10 f(3)= 21 4+2( 21)- 10= 36 f(4)= 36
5 f(3)= 21 f(4)= 36 4+2( 36)- 21= 55 f(5)= 55
b

We will use patterns between consecutive data pairs to determine what type of function best models the given values from Part A. The differences of consecutive f-values are called first differences. The differences of consecutive first differences are called second differences.

  • Linear Function: The first differences are constant.
  • Quadratic Function: The second differences are constant.
  • Exponential Function: Consecutive y-values have a common ratio.

    Remember that in all cases the differences of consecutive n-values need to be constant! Let's analyze the table of values and compute the first differences.

    Since the first differences are not constant, the sequence cannot be modeled by a linear function. Next we will check the second differences.

    Since the second differences are constant, the sequence can be modeled by a quadratic function.

c From Part B we know that the sequence can be modeled by a quadratic function, f(n)=an^2+bn+c, for some values of a, b, and c. To determine the values of a, b, and c, let's substitute 1, 2, and 3 for n and use that f( 1)= 3, f( 2)= 10, f( 3)= 21.
f(n)=an^2+bn+c ⇓ 3=a( 1)^2+b( 1)+c & (I) 10=a( 2)^2+b( 2)+c & (II) 21=a( 3)^2+b( 3)+c & (III) Let's use the Substitution Method to find a solution to this system.
3=a(1)^2+b(1)+c & (I) 10=a(2)^2+b(2)+c & (II) 21=a(3)^2+b(3)+c & (III)
â–Ľ
Solve by substitution

(I), (II), (III):Calculate power and product

3=a+b+c & (I) 10=4a+2b+c & (II) 21=9a+3b+c & (III)

(I), (II), (III):Rearrange equation

a+b+c=3 & (I) 4a+2b+c=10 & (II) 9a+3b+c=21 & (III)
b+c=3-a & (I) 4a+2b+c=10 & (II) 9a+3b+c=21 & (III)
c=3-a-b & (I) 4a+2b+c=10 & (II) 9a+3b+c=21 & (III)

(I), (II):c= 3-a-b

c=3-a-b & (I) 4a+2b+ 3-a-b=10 & (II) 9a+3b+ 3-a-b=21 & (III)

(II), (III): Add and subtract terms

c=3-a-b & (I) 3a+b+3=10 & (II) 8a+2b+3=21 & (III)

(II), (III): LHS-3=RHS-3

c=3-a-b & (I) 3a+b=7 & (II) 8a+2b=18 & (III)
c=3-a-b & (I) b=7-3a & (II) 8a+2b=18 & (III)
c=3-a-b & (I) b=7-3a & (II) 8a+2( 7-3a)=18 & (III)
c=3-a-b & (I) b=7-3a & (II) 8a+14-6a=18 & (III)
c=3-a-b & (I) b=7-3a & (II) 2a+14=18 & (III)
c=3-a-b & (I) b=7-3a & (II) 2a=4 & (III)
c=3-a-b & (I) b=7-3a & (II) a=2 & (III)
c=3-a-b & (I) b=7-3( 2) & (II) a=2 & (III)
c=3-a-b & (I) b=7-6 & (II) a=2 & (III)
c=3-a-b & (I) b=1 & (II) a=2 & (III)
c=3- 2- 1 & (I) b=1 & (II) a=2 & (III)
c=0 & (I) b=1 & (II) a=2 & (III)
Therefore, a= 2, b= 1, and c= 0. Now, let's substitute the values into the equation f(n)= an^2+ bn+ c.
f(n)= an^2+ bn+ c
â–Ľ
Substitute values and simplify
f(n)= 2n^2+ 1n+ 0
f(n)=2n^2+1n
f(n)=2n^2+n
Finally, an explicit rule for the given sequence is f(n)=2n^2+n.

Checking the Answer

Since we obtain the explicit rule f(n)=2n^2+n based only on a few first terms of the given sequence, we are not sure if this is correct for all values of n. To verify our answer, we should substitute f(n)=2n^2+n into the recursive rule for f(n) and check if it is true. f(n)=4+2f(n-1)-f(n-2) First, let's find f(n-1) and f(n-2). f( n)=2 n^2+ n ⇓ f( n-1)=2( n-1)^2+( n-1) f( n-2)=2( n-2)^2+( n-2) Next, we will substitute 2n^2+n for f(n), 2(n-1)^2+(n-1) for f(n-1), and 2(n-2)^2+(n-2) for f(n-2) into the recursive formula.
f(n)=4+2 f(n-1)-f(n-2)
â–Ľ
Substitute values and simplify
2n^2+n? =4+2( 2(n-1)^2+(n-1))-(2(n-2)^2+(n-2))
2n^2+n? =4+2(2(n-1)^2+n-1)-2(n-2)^2-n+2
2n^2+n? =4+2(2(n^2-2n+1)+n-1)-2(n^2-4n+4)-n+2
2n^2+n? =4+2(2n^2-4n+2+n-1)-2n^2+8n-8-n+2
2n^2+n? =2(2n^2-3n+1)-2n^2+7n-2
2n^2+n? =4n^2-6n+2-2n^2+7n-2
2n^2+n? =2n^2+n
2n^2? =2n^2
0=0 âś“
Substitution produces a true statement. This tells us that the explicit formula f(n)=2n^2+n is true for all values of n.