Question

Given: 1; -5; -13 ; -23 ; ... Derive a formula for the nth term in the pattern.

Answer 1

Answer:

  f(n) = -n^2 -3n +5

Step-by-step explanation:

Suppose the formula is ...

  f(n) = an^2 +bn +c

Then we have ...

  f(1) = 1 = a(1^2) +b(1) +c

  f(2) = -5 = a(2^2) +b(2) +c

  f(3) = -13 = a(3^2) +b(3) +c

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Here's a way to solve these equations.

Subtract the first equation from the second:

  -6 = 3a +b . . . . . 4th equation

Subtract the second equation from the third:

  -8 = 5a +b . . . . . 5th equation

Subtract the fourth equation from the fifth:

  -2 = 2a

  a = -1

Then substituting into the 4th equation to find b, we have ...

  -6 = 3(-1) +b

  -3 = b

and ...

  1 = -1 +(-3) +c . . . . . substituting "a" and "b" into the first equation

  5 = c

The formula is ...

  f(n) = -n^2 -3n +5