Mathematics

Problem

The curve

x=\sin t,\; y=\cos t

(

0\le t<2\pi

) has a vertical tangent when a)

t=0\text{ or }\pi

t=\dfrac{\pi}{4}\text{ or }\dfrac{5\pi}{4}

t=\dfrac{\pi}{2}\text{ or }\dfrac{3\pi}{2}

t=\dfrac{3\pi}{4}\text{ or }\dfrac{7\pi}{4}