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Polynomial Function for Triangular Pyramidal Numbers

Learn how to find the nth triangular pyramidal number using a polynomial function with examples and solutions. Explore finite differences and matrix equations to derive the function.

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Polynomial Function for Triangular Pyramidal Numbers

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  1. The first seven triangular pyramidal numbers are shown below. Find a polynomial function that gives the nth triangular pyramidal number. EXAMPLE 3 Model with finite differences SOLUTION Begin by finding the finite differences.

  2. EXAMPLE 3 Model with finite differences Because the third-order differences are constant, you know that the numbers can be represented by a cubic function of the form f (n) = an3 + bn2 + cn + d. By substituting the first four triangular pyramidal numbers into the function, you obtain a system of four linear equations in four variables.

  3. a + b + c + d = 1 8a + 4b + 2c + d = 4 27a + 9b + 3c + d = 10 64a + 16b + 4c + d = 20 Model with finite differences EXAMPLE 3 a(1)3 + b(1)2 + c(1) + d = 1 a(2)3 + b(2)2 + c(2) + d = 4 a(3)3 + b(3)2 + c(3) + d = 10 a(4)3 + b(4)2 + c(4) + d = 20 Write the linear system as a matrix equation AX = B. Enter the matrices Aand Binto a graphing calculator, and then calculate the solution X = A– 1 B.

  4. a b c d 1 4 10 20 1 8 27 64 1 4 9 16 1 2 3 4 1 1 1 1 = A x B 1 1 1 The solution is a = ,b = , c = , andd = 0. So, the nth triangular pyramidal number is given by f (n) = n3 + n2 + n. 3 6 2 1 1 1 2 3 6 Model with finite differences EXAMPLE 3

  5. Use finite differences to find a polynomial function • that fits the data in the table. Write function values for equally-spaced n - values. First-order differences Second-order differences Third-order differences for Example 3 GUIDED PRACTICE SOLUTION

  6. a + b + c + d = 6 8a + 4b + 2c + d = 15 27a + 9b + 3c + d = 22 64a + 16b + 4c + d = 21 for Example 3 GUIDED PRACTICE Because the third-order differences are constant, you know that the numbers can be represented by a cubic function of the form f (n) = an3 + bn2 + cn + d. By substituting the first four triangular pyramidal numbers into the function, you obtain a system of four linear equations in four variables. a(1)3 + b(1)2 + c(1) + d = 6 a(2)3 + b(2)2 + c(2) + d = 15 a(3)3 + b(3)2 + c(3) + d = 22 a(4)3 + b(4)2 + c(4) + d = 1

  7. a b c d 6 15 22 21 1 8 27 64 1 4 9 16 1 2 3 4 1 1 1 1 = for Example 3 GUIDED PRACTICE Write the linear system as a matrix equation AX = B. Enter the matrices Aand Binto a graphing calculator, and then calculate the solution X = A– 1 B. The solution is a =1, b =5, c =1, andd = 1. So, the nth triangular pyramidal number is given by f (x) = – x3 + 5x2 + x + 1.

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