The problem is first solve for scalar field \(\theta \) with the interpolating polynomial \(a_{1}+a_{2}x+a_{3}y\). Writing\begin {equation} \theta =\begin {bmatrix} 1 & x & y \end {bmatrix}\begin {bmatrix} a_{1}\\ a_{2}\\ a_{3}\end {bmatrix} \tag {1} \end {equation} Evaluating the field \(\theta \) at each node gives\[\begin {bmatrix} \theta _{1}\\ \theta _{2}\\ \theta _{3}\end {bmatrix} =\begin {bmatrix} 1 & x_{1} & y_{1}\\ 1 & x_{2} & y_{2}\\ 1 & x_{3} & y_{3}\end {bmatrix}\begin {bmatrix} a_{1}\\ a_{2}\\ a_{3}\end {bmatrix} \] Hence\begin {align} \begin {bmatrix} a_{1}\\ a_{2}\\ a_{3}\end {bmatrix} & =\begin {bmatrix} 1 & x_{1} & y_{1}\\ 1 & x_{2} & y_{2}\\ 1 & x_{3} & y_{3}\end {bmatrix} ^{-1}\begin {bmatrix} \theta _{1}\\ \theta _{2}\\ \theta _{3}\end {bmatrix} \nonumber \\ & =\frac {1}{\Delta }\begin {bmatrix} x_{2}y_{3}-x_{3}y_{2} & x_{3}y_{1}-x_{1}y_{3} & x_{1}y_{2}-x_{2}y_{1}\\ y_{2}-y_{3} & y_{3}-y_{1} & y_{1}-y_{2}\\ x_{3}-x_{2} & x_{1}-x_{3} & x_{2}-x_{1}\end {bmatrix}\begin {bmatrix} \theta _{1}\\ \theta _{2}\\ \theta _{3}\end {bmatrix} \tag {2} \end {align}
Where \(\Delta \) is the determinant \(x_{1}y_{2}-x_{2}y_{1}-x_{1}y_{3}+x_{3}y_{1}+x_{2}y_{3}-x_{3}y_{2}\). Substituting (2) into (1) gives\begin {align} \theta & =\overbrace {\frac {1}{\Delta }\begin {bmatrix} 1 & x & y \end {bmatrix}\begin {bmatrix} x_{2}y_{3}-x_{3}y_{2} & x_{3}y_{1}-x_{1}y_{3} & x_{1}y_{2}-x_{2}y_{1}\\ y_{2}-y_{3} & y_{3}-y_{1} & y_{1}-y_{2}\\ x_{3}-x_{2} & x_{1}-x_{3} & x_{2}-x_{1}\end {bmatrix} }\begin {bmatrix} \theta _{1}\\ \theta _{2}\\ \theta _{3}\end {bmatrix} \nonumber \\ & =\begin {bmatrix} N_{1} & N_{2} & N_{3}\end {bmatrix}\begin {bmatrix} \theta _{1}\\ \theta _{2}\\ \theta _{3}\end {bmatrix} \tag {3} \end {align}
Where \begin {align} N_{1} & =\frac {1}{\Delta }\left [ x_{2}y_{3}-x_{3}y_{2}+x\left ( y_{2}-y_{3}\right ) +y\left ( x_{3}-x_{2}\right ) \right ] \tag {4}\\ N_{2} & =\frac {1}{\Delta }\left [ x_{3}y_{1}-x_{1}y_{3}+x\left ( y_{3}-y_{1}\right ) +y\left ( x_{1}-x_{3}\right ) \right ] \nonumber \\ N_{3} & =\frac {1}{\Delta }\left [ x_{1}y_{2}-x_{2}y_{1}+x\left ( y_{1}-y_{2}\right ) +y\left ( x_{2}-x_{1}\right ) \right ] \nonumber \end {align}
For constant stress triangle, the field is a vector field. Hence replacing \(\theta \) with \(\begin {bmatrix} u\\ v \end {bmatrix} \) equation (3) becomes\[\begin {bmatrix} u\\ v \end {bmatrix} =\begin {bmatrix} N_{1} & 0 & N_{2} & 0 & N_{3} & 0\\ 0 & N_{1} & 0 & N_{2} & 0 & N_{3}\end {bmatrix}\begin {bmatrix} u_{1}\\ v_{1}\\ u_{2}\\ v_{2}\\ u_{3}\\ v_{3}\end {bmatrix} \] From the above \begin {align*} \frac {\partial u}{\partial x} & =\frac {\partial N_{1}}{\partial x}u_{1}+\frac {\partial N_{2}}{\partial x}u_{2}+\frac {\partial N_{3}}{\partial x}u_{3}\\ \frac {\partial v}{\partial y} & =\frac {\partial N_{1}}{\partial y}v_{1}+\frac {\partial N_{2}}{\partial y}v_{2}+\frac {\partial N_{3}}{\partial y}v_{3}\\ \frac {\partial u}{\partial y}+\frac {\partial v}{\partial x} & =\frac {\partial N_{1}}{\partial y}u_{1}+\frac {\partial N_{2}}{\partial y}u_{2}+\frac {\partial N_{3}}{\partial y}u_{3}+\frac {\partial N_{1}}{\partial x}v_{1}+\frac {\partial N_{2}}{\partial x}v_{2}+\frac {\partial N_{3}}{\partial x}v_{3} \end {align*}
Hence\begin {align} \begin {bmatrix} \varepsilon _{x}\\ \varepsilon _{y}\\ \gamma _{xy}\end {bmatrix} & =\begin {bmatrix} \frac {\partial }{\partial x} & 0\\ 0 & \frac {\partial }{\partial y}\\ \frac {\partial }{\partial y} & \frac {\partial }{\partial x}\end {bmatrix}\begin {bmatrix} u\\ v \end {bmatrix} \nonumber \\ & =\begin {bmatrix} \frac {\partial }{\partial x}u\\ \frac {\partial }{\partial y}v\\ \frac {\partial }{\partial y}u+\frac {\partial }{\partial x}v \end {bmatrix} \nonumber \\ & =\begin {bmatrix} \frac {\partial N_{1}}{\partial x}u_{1}+\frac {\partial N_{2}}{\partial x}u_{2}+\frac {\partial N_{3}}{\partial x}u_{3}\\ \frac {\partial N_{1}}{\partial y}v_{1}+\frac {\partial N_{2}}{\partial y}v_{2}+\frac {\partial N_{3}}{\partial y}v_{3}\\ \frac {\partial N_{1}}{\partial y}u_{1}+\frac {\partial N_{2}}{\partial y}u_{2}+\frac {\partial N_{3}}{\partial y}u_{3}+\frac {\partial N_{1}}{\partial x}v_{1}+\frac {\partial N_{2}}{\partial x}v_{2}+\frac {\partial N_{3}}{\partial x}v_{3}\end {bmatrix} \nonumber \\ & =\overset {B}{\overbrace {\begin {bmatrix} \frac {\partial N_{1}}{\partial x} & 0 & \frac {\partial N_{2}}{\partial x} & 0 & \frac {\partial N_{3}}{\partial x} & 0\\ 0 & \frac {\partial N_{1}}{\partial y} & 0 & \frac {\partial N_{2}}{\partial y} & 0 & \frac {\partial N_{3}}{\partial y}\\ \frac {\partial N_{1}}{\partial y} & \frac {\partial N_{1}}{\partial x} & \frac {\partial N_{2}}{\partial y} & \frac {\partial N_{2}}{\partial x} & \frac {\partial N_{3}}{\partial y} & \frac {\partial N_{3}}{\partial x}\end {bmatrix} }}\begin {bmatrix} u_{1}\\ v_{1}\\ u_{2}\\ v_{2}\\ u_{3}\\ v_{3}\end {bmatrix} \tag {5} \end {align}
From (4) all of the \(\frac {\partial N_{i}}{\partial x},\frac {\partial N_{j}}{\partial y}\)terms are evaluated. Substituting the result into (5) gives the \(B\) matrix\begin {align*} \frac {\partial N_{1}}{\partial x} & =\frac {1}{\Delta }\left ( y_{2}-y_{3}\right ) \\ \frac {\partial N_{2}}{\partial x} & =\frac {1}{\Delta }\left ( y_{3}-y_{1}\right ) \\ \frac {\partial N_{3}}{\partial x} & =\frac {1}{\Delta }\left ( y_{1}-y_{2}\right ) \end {align*}
And\begin {align*} \frac {\partial N_{1}}{\partial y} & =\frac {1}{\Delta }\left ( x_{3}-x_{2}\right ) \\ \frac {\partial N_{2}}{\partial y} & =\frac {1}{\Delta }\left ( x_{1}-x_{3}\right ) \\ \frac {\partial N_{3}}{\partial y} & =\frac {1}{\Delta }\left ( x_{2}-x_{1}\right ) \end {align*}
Hence \(B\) becomes\begin {equation} B=\frac {1}{\Delta }\begin {bmatrix} y_{2}-y_{3} & 0 & y_{3}-y_{1} & 0 & y_{1}-y_{2} & 0\\ 0 & x_{3}-x_{2} & 0 & x_{1}-x_{3} & 0 & x_{2}-x_{1}\\ x_{3}-x_{2} & y_{2}-y_{3} & x_{1}-x_{3} & y_{3}-y_{1} & x_{2}-x_{1} & y_{1}-y_{2}\end {bmatrix} \tag {6} \end {equation} Letting \(y_{i}-y_{j}=y_{ij}\) and \(x_{i}-x_{j}=x_{ij}\), the above becomes\[ B=\frac {1}{\Delta }\begin {bmatrix} y_{23} & 0 & y_{31} & 0 & y_{12} & 0\\ 0 & x_{32} & 0 & x_{13} & 0 & x_{21}\\ x_{32} & y_{23} & x_{13} & y_{31} & x_{21} & y_{12}\end {bmatrix} \] But the area of triangle is given by\begin {align*} A & =\frac {1}{2}\begin {vmatrix} \mathbf {i} & \mathbf {j} & \mathbf {k}\\ x_{2}-x_{1} & y_{2}-y_{1} & 0\\ x_{3}-x_{1} & y_{3}-y_{1} & 0 \end {vmatrix} \\ 2A & =\left ( x_{2}-x_{1}\right ) \left ( y_{3}-y_{1}\right ) -\left ( y_{2}-y_{1}\right ) \left ( x_{3}-x_{1}\right ) \\ & =x_{1}y_{2}-x_{2}y_{1}-x_{1}y_{3}+x_{3}y_{1}+x_{2}y_{3}-x_{3}y_{2} \end {align*}
And the determinant \(\Delta \) was found above to be \(x_{1}y_{2}-x_{2}y_{1}-x_{1}y_{3}+x_{3}y_{1}+x_{2}y_{3}-x_{3}y_{2}\), hence\[ 2A=\Delta \] Substituting the above into \(B\) found above in equation (6) gives\[ B=\frac {1}{2A}\begin {bmatrix} y_{2}-y_{3} & 0 & y_{3}-y_{1} & 0 & y_{1}-y_{2} & 0\\ 0 & x_{3}-x_{2} & 0 & x_{1}-x_{3} & 0 & x_{2}-x_{1}\\ x_{3}-x_{2} & y_{2}-y_{3} & x_{1}-x_{3} & y_{3}-y_{1} & x_{2}-x_{1} & y_{1}-y_{2}\end {bmatrix} \]