[tex]\Large\boxed{\begin{array}{l}\sf se~x\in~2^o~quadrante, cos(x)<0.\\\sf sen(x)=\dfrac{\sqrt{2}}{2}\implies sen^2(x)=\dfrac{2\div2}{4\div2}=\dfrac{1}{2}\\\\\sf\dfrac{2}{2}-\dfrac{1}{2}=\dfrac{1}{2}\\\\\sf cos^2(x)=\dfrac{1}{2}\\\\\sf cos(x)=-\sqrt{\dfrac{1}{2}}\\\\\sf cos(x)=-\dfrac{1}{\sqrt{2}}\\\\\sf cos(x)=-\dfrac{\sqrt{2}}{2}\end{array}}[/tex]
