Source TeX:

\begin{array}{rll}\sqrt{2}\sin(x)&=\dfrac{\sin(x)}{\cos(x)}&\\[1em]\sqrt{2}\sin(x)\cos(x)&=\sin(x)&\small\text{; on a multiplié chaque}\\&&\small\text{ membre par }\cos(x)\\\sqrt{2}\sin(x)\cos(x)-\sin(x)&=0&\\[1em]\underset{(1)}{\underbrace{\sin(x)}}\underset{(2)}{\underbrace{\left(\sqrt{2}\cos(x)-1\right)}}&=0&\small\text{; on a factorisé}\end{array}