Which one of the following expressions gives the area of the triangle with vertices at (x1,y1), (x2,y2) and (x3,y3)?
(a) 21(x1(y2−y3)+x2(y3−y1)+x3(y1−y2))
(b) 21∣x1(y2−y3)−x2(y3−y1)+x3(y1−y2)∣
(c) 21∣x1(y2−y3)+x2(y3−y1)+x3(y1−y2)∣
(d) ∣x1(y2−y3)+x2(y3−y1)+x3(y1−y2)∣