# How do you use the binomial series to expand #(x+1+x^-1)^4#?

##### 1 Answer

#=x^4+4x^3+10x^2+16x+19+16x^(-1)+10x^(-2)+4x^(-3)+x^(-4)#

#### Explanation:

This is a trinomial, not a binomial. With binomials you could use Pascal's triangle to give you the coefficients.

Since the terms of the given trinomial are in geometric progression, we can use coefficients from a generalisation of Pascal's triangle that adds together

#color(white)(00000000000000)1#

#color(white)(00000000000)1color(white)(00)1color(white)(00)1#

#color(white)(00000000)1color(white)(00)2color(white)(00)3color(white)(00)2color(white)(00)1#

#color(white)(00000)1color(white)(00)3color(white)(00)6color(white)(00)7color(white)(00)6color(white)(00)3color(white)(00)1#

#color(white)(00)1color(white)(00)4color(white)(0)10color(white)(0)16color(white)(0)19color(white)(0)16color(white)(0)10color(white)(00)4color(white)(00)1#

In this triangle each number is the sum of the three numbers above it: left, centre and right.

Hence we find:

#(x+1+x^(-1))^4#

#=x^4+4x^3+10x^2+16x+19+16x^(-1)+10x^(-2)+4x^(-3)+x^(-4)#