Multinomial theorem

(x1+x2)ⁿ = Σ (n!/r1!r2!) x1^r1x2^r2 (where (r1 = 0 to n) and r1+r2 = n)


The result can be extended to (x1+x2_x3+…+xk) ⁿ

(x1+x2_x3+…+xk) ⁿ =
Σ [n!/(r1!r2!...rk!)] x1^r1x2^r2 xk^rk (where r1+r2+…+rn = n)