[질문] 세점을이용해정삼각형넓이구하는공식?

ABC의넓이구할때A(x1,y1),B(x2,y2),C(x3,y3)에서|x₁,x₂,x₃,x₁||y₁,y₂,y₃,y₁|로적어서삼각형넓이는1/2(|x₁y₂+x₂y₃+x₃y₁|-|y₁x₂+y₂x₃+y₃x₁|)