[tex]\cos\dfrac{3\pi}{4}=\cos\left(\pi-\dfrac{\pi}{4}\right)=-\cos\dfrac{\pi}{4}=-\dfrac{\sqrt2}{2}[/tex]
[tex]\sin\dfrac{3\pi}{4}=\sin\left(\pi-\dfrac{\pi}{4}\right)=\sin\dfrac{\pi}{4}=\dfrac{\sqrt2}{3}[/tex]
Look at the picture.
[tex]\dfrac{\pi}{2} \ \textless \ \dfrac{3\pi}{4} \ \textless \ \pi\\\\therefoere\\\\\cos\dfrac{3\pi}{4} \ \textless \ 0\ and\ \sin\dfrac{3\pi}{4} \ \textgreater \ 0[/tex]
