We know,
[tex]\cos (-\theta)=\cos \theta[/tex]Therefore,
[tex]\cos (-90^{\circ})=\cos 90^{\circ}[/tex]We also know,
[tex]\cos 90^{\circ}=0[/tex]Hence,
[tex]\begin{gathered} \cos (-90^{\circ})=\cos 90^{\circ}=0 \\ \cos (-90^{\circ})=0 \end{gathered}[/tex]So, the value of cos(-90) is zero.
