Question 1101896

Alan,
What is this process called? What is the rational behind it?

x -3,2,8,-3
y 2,7,10,2
add 
-3*7+2*10+8*2
-21+20+16=15
add
2*2+7*8+(10*-3)=30
4+56-30=30
subtract
15-30=-15
1/2*|-15|=7.5 sq units
{{{drawing(400,400,-4,11,0,11, graph(400,400,0,11,0,11), grid(1),
locate(-3,2,"(-3,2)"), locate(2, 7,"(2, 7)"), locate(8, 10,"(8, 10)"),
locate(-3.5,2,"A"), locate(2, 7.5,"B"), locate(8, 10.5,"C"),
triangle(-3,2,2, 7,8, 10)   )}}}