## Calculus 8th Edition

The parametric equations of a circle whose radius is $r$ are given as follows: $x=r \cos t ; y =r \sin t$ From the given problem, we have $x= t \cos t , y= \sin t , z=\dfrac{1}{1+t^2}$ When we look down from a upper view with the high $z$-value then, we get a circle and the parametric equation $z=\dfrac{1}{1+t^2}$ shows that $z$ is always positive. The above conclusion signifies the conditions indicated in the Graph{(VI).