Then shake hands X(X- 1) times.

However, since the handshake between A and B is the same as that between B and A, this number must be divided by 2.

So X(X- 1)÷2=28.

Extended to 1/2X? - 1/2X-28=0

x？ -X-56=0

Factorization gives (X-8)(X+7)=0.

The solution is X 1=8, X2=-7 (irrelevant, omitted).

∴X=8

So eight people attended the party.