Set of points of solid triangle in plane

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Consider a triangle in the plane with vertices $a,b,c$ (viewed as vectors). I'm struggling to understand why the region of the solid triangle is given by $\{t_1a+t_2b+t_3c:t_i\in [0,1], \Sigma_{i=1}^{3}t_i=1\}$. The inside looks like a dot product of sorts, but I'm not sure.