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.