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½(x2-x1)(y1+y2)
½(x3-x2)(y2+y3)
½(x1-x3)(y3+y1)
1
2
3
1
2
3
1
2
3
x1
x2
x2
x3
x1
x3
½(y2-y1)(x1+x2)
½(y3-y2)(x2+x3)
½(y1-y3)(x3+x1)
1
2
3
1
2
3
1
2
3
y1
y2
y2
y3
y1
y3