Parametric Equation Question Ok this is a really silly question and I should know this, but I can't seem to figure something out:
for the last step, how do they know that $0 \leq x \leq 4$? If we use the minimum value of theta, which is $-\pi/2$, and plug that into $x=\cos(\theta)$, then we get $0$, and same for $\pi/2$.

Because over $-\pi /2\leq \theta \leq \pi /2, 0\leq\cos{\theta}\leq1$.  At $\theta =0, \cos{\theta}=1$
