多角形の面積、多角形の重心

//aに面積が求まる double[] x = {4,4,2,2}; double[] y = {2,4,4,2}; double a = 0; for (int i = x.length,u = 0; i-->0;u=i) a += (x[i]-x[u])*(y[i]+y[u]); a = Math.abs(0.5*a); //aに面積,cx,cyに重心座標が求まる double[] x = {4,4,2,2}; double[] y = {2,4,4,2}; double a = 0, cx = 0, cy = 0; for (int i = x.length,u = 0; i-->0;u=i) { double k = x[i]*y[u] - x[u]*y[i]; a += k; cx += k*(x[u]+x[i]); cy += k*(y[u]+y[i]); } cx /= 3.