For vertices (x1,y1), (x2,y2) and (x3,y3), which of the following formulas represents the area A of the triangle they form?
(a) ∣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) 21∣x1(y2+y3)+x2(y3+y1)+x3(y1+y2)∣