Question

when using the binomial formula which of the following is the correct step in the expansion of (x+y)^4

Answer 1

The binomial formula is the following:
$(x+y)^4=\Sigma_{k=0}^4C_4^kx^ky^{4-k}\\=y^4+xy^3C_4^1+x^2y^2C_4^2\\+x^3yC_4^2+x^4\\=y^4+4xy^3+6x^2y^2+4x^3y+x^4$
We have used the formula:
$C_4^k=\dfrac{4!}{(4-k)!k!}$
k goes from 0 to 4.