Sigma + binomial coefficient?

1.= ) Insert the definition of Bin.koeff.

2.= Once I’ve done a little bit through an intermediate step, hope that’s understandable.

n! you can (if n is large enough) also write. The red-marked terms can be shortened, the green-marked is only a 0, so the top is n*(n-1) and below 2!=2*1=2

3.= ) 1/2 can be drawn as a factor before the sum, so the denominator falls away, in the counter the clamp is only multiplied.

Furthermore, the sum sign is then resolved by inserting explicit write-up of the sum. The remainder is calculation of the terms.