for a, b, c, d, e, f, g, h, i being Real holds |{|[a,b,c]|,|[d,e,f]|,|[g,h,i]|}| = ((((((a * e) * i) + ((b * f) * g)) + ((c * d) * h)) - ((g * e) * c)) - ((h * f) * a)) - ((i * d) * b)