In Wikipedia https://en.wikipedia.org/wiki/Stiffness_matrix under section "Practical assembly of the stiffness matrix" section, it says that:
For example, for piecewise linear elements, consider a triangle with vertices $(x_1, y_1), (x_2, y_2), (x_3, y_3)$, and define the 2×3 matrix $$D = \begin{bmatrix}x_3 -x_2 & x_1-x_3&x_2-x_1\\ y_3-y_2& y_1-y_3& y_2-y_1 \end{bmatrix}$$ Then the element stiffness matrix is $$A^{[k]} = \frac{D^TD}{4 area(T)}$$ I wonder how the above expression, the element stiffness matrix for a triangle, is derived.
Thank you.