//■X軸とY軸からZ軸を求める

    Vector3 x_axis = new Vector3(1f, 0f, 0f);
    Vector3 y_axis = new Vector3(0f, 1f, 0f);

    //■外積

    Vector3 z_axis = Vector3.Cross(x_axis, y_axis);           

    //--------------------------------------------------


    //■各成分を公式に従って計算してみる

    z_axis.X = (x_axis.Y * y_axis.Z) - (x_axis.Z * y_axis.Y);
    z_axis.Y = (x_axis.Z * y_axis.X) - (x_axis.X * y_axis.Z);
    z_axis.Z = (x_axis.X * y_axis.Y) - (x_axis.Y * y_axis.X);