Question 1084977
You can use the points themselves,
{{{2A=abs(x[1](y[2]-y[3])+x[2](y[3]-y[1])+x[3](y[1]-y[2]))}}}
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You can use the distance formula to find the lengths of the sides of the triangle (a,b,c) and then use Heron's formula,
{{{A=sqrt(s(s-a)(s-b)(s-c))}}}
where
{{{s=(a+b+c)/2}}}