# How do you find the explicit formula for the following sequence 2, -4, -16, -36......?

##### 1 Answer

Examine sequences of differences to derive a polynomial formula for the sequence:

#a_n = -1/3n^3-n^2-2/3n+4#

#### Explanation:

A finite number of terms does not determine a unique explicit formula, but we can find a formula that will work.

Start with the given sequence:

#color(blue)(2), -4, -16, -36#

Form the sequence of differences of that sequence:

#color(blue)(-6), -12, -20#

Form the sequence of differences of that sequence:

#color(blue)(-6), -8#

Form the sequence of differences of that sequence:

#color(blue)(-2)#

We can construct a polynomial formula for

#a_n = color(blue)(2)/(0!)+color(blue)(-6)/(1!)(n-1)+color(blue)(-6)/(2!)(n-1)(n-2)+color(blue)(-2)/(3!)(n-1)(n-2)(n-3)#

#=2-6n+6-3n^2+9n-6-1/3n^3+2n^2-11/3n+2#

#=-1/3n^3-n^2-2/3n+4#