Trinomial expansion
# Trinomial expansion

In mathematics, a trinomial expansion is the expansion of a power of a sum of three terms into monomials. The expansion is given by

    $(a+b+c)^{n}=\sum _{{i,j,k} \atop {i+j+k=n}}{n \choose i,j,k}\,a^{i}\,b^{\;\!j}\;\!c^{k},$

where n is a nonnegative integer and the sum is taken over all combinations of nonnegative indices i, j, and k such that i + j + k = n. The trinomial coefficients are given by

    ${n \choose i,j,k}={\frac {n!}{i!\,j!\,k!}}\,.$

This formula is a special case of the multinomial formula for m = 3. The coefficients can be defined with a generalization of Pascal's triangle to three dimensions, called Pascal's pyramid or Pascal's tetrahedron.

## Derivation

The trinomial expansion can be calculated by applying the binomial expansion twice, setting $d=b+c$, which leads to

    ${\begin{aligned}(a+b+c)^{n}&=(a+d)^{n}=\sum _{r=0}^{n}{n \choose r}\,a^{n-r}\,d^{r}\\&=\sum _{r=0}^{n}{n \choose r}\,a^{n-r}\,(b+c)^{r}\\&=\sum _{r=0}^{n}{n \choose r}\,a^{n-r}\,\sum _{s=0}^{r}{r \choose s}\,b^{r-s}\,c^{s}.\end{aligned}}$

Above, the resulting $(b+c)^{r}$ in the second line is evaluated by the second application of the binomial expansion, introducing another summation over the index $s$.

The product of the two binomial coefficients is simplified by shortening $r!$,

    ${n \choose r}\,{r \choose s}={\frac {n!}{r!(n-r)!}}{\frac {r!}{s!(r-s)!}}={\frac {n!}{(n-r)!(r-s)!s!}},$

and comparing the index combinations here with the ones in the exponents, they can be relabelled to $i=n-r,~j=r-s,~k=s$, which provides the expression given in the first paragraph.

## Properties

The number of terms of an expanded trinomial is the triangular number

    $t_{n+1}={\frac {(n+2)(n+1)}{2}},$

where n is the exponent to which the trinomial is raised.

## Example

An example of a trinomial expansion with $n=2$ is :

$(a+b+c)^{2}=a^{2}+b^{2}+c^{2}+2ab+2bc+2ca$

