$$
\newcommand{\uex}{{u_{\small\mbox{e}}}}
\newcommand{\Aex}{{A_{\small\mbox{e}}}}
\newcommand{\half}{\frac{1}{2}}
\newcommand{\tp}{\thinspace .}
\newcommand{\Oof}[1]{\mathcal{O}(#1)}
\newcommand{\x}{\boldsymbol{x}}
\newcommand{\X}{\boldsymbol{X}}
\renewcommand{\u}{\boldsymbol{u}}
\renewcommand{\v}{\boldsymbol{v}}
\newcommand{\e}{\boldsymbol{e}}
\newcommand{\f}{\boldsymbol{f}}
\newcommand{\dfc}{\alpha} % diffusion coefficient
\newcommand{\Ix}{\mathcal{I}_x}
\newcommand{\Iy}{\mathcal{I}_y}
\newcommand{\Iz}{\mathcal{I}_z}
\newcommand{\If}{\mathcal{I}_s} % for FEM
\newcommand{\Ifd}{{I_d}} % for FEM
\newcommand{\Ifb}{{I_b}} % for FEM
\newcommand{\sequencei}[1]{\left\{ {#1}_i \right\}_{i\in\If}}
\newcommand{\basphi}{\varphi}
\newcommand{\baspsi}{\psi}
\newcommand{\refphi}{\tilde\basphi}
\newcommand{\psib}{\boldsymbol{\psi}}
\newcommand{\sinL}[1]{\sin\left((#1+1)\pi\frac{x}{L}\right)}
\newcommand{\xno}[1]{x_{#1}}
\newcommand{\Xno}[1]{X_{(#1)}}
\newcommand{\xdno}[1]{\boldsymbol{x}_{#1}}
\newcommand{\dX}{\, \mathrm{d}X}
\newcommand{\dx}{\, \mathrm{d}x}
\newcommand{\ds}{\, \mathrm{d}s}
$$
Integration over a reference element; element matrix
P1 elements and \( f(x)=x(1-x) \).
$$
\begin{align}
\tilde A^{(e)}_{0,0}
&= \int_{-1}^1 \refphi_0(X)\refphi_0(X)\frac{h}{2} dX\nonumber\\
&=\int_{-1}^1 \half(1-X)\half(1-X) \frac{h}{2} dX =
\frac{h}{8}\int_{-1}^1 (1-X)^2 dX = \frac{h}{3}
\tag{6}\\
\tilde A^{(e)}_{1,0}
&= \int_{-1}^1 \refphi_1(X)\refphi_0(X)\frac{h}{2} dX\nonumber\\
&=\int_{-1}^1 \half(1+X)\half(1-X) \frac{h}{2} dX =
\frac{h}{8}\int_{-1}^1 (1-X^2) dX = \frac{h}{6}\\
\tilde A^{(e)}_{0,1} &= \tilde A^{(e)}_{1,0}
\tag{7}\\
\tilde A^{(e)}_{1,1}
&= \int_{-1}^1 \refphi_1(X)\refphi_1(X)\frac{h}{2} dX\nonumber\\
&=\int_{-1}^1 \half(1+X)\half(1+X) \frac{h}{2} dX =
\frac{h}{8}\int_{-1}^1 (1+X)^2 dX = \frac{h}{3}
\tag{8}
\end{align}
$$
