#### Answer

a) $x(0)=10(cm)$, $x(\pi/3)=5(cm)$ and $x(3\pi/4)=-5\sqrt2(cm)$
b) $v(0)=0(cm/s)$, $v(\pi/3)=-5\sqrt3(cm/s)$ and $v(3\pi/4)=-5\sqrt2(cm/s)$.

#### Work Step by Step

$$x=10\cos t$$
a) For $t=0$: $$x=10\cos0=10\times1=10(cm)$$
- For $t=\pi/3$: $$x=10\cos\frac{\pi}{3}=10\times\frac{1}{2}=5(cm)$$
- For $t=3\pi/4$: $$x=10\cos\frac{3\pi}{4}=10\times\Big(-\frac{\sqrt2}{2}\Big)=-5\sqrt2(cm)$$
b) First, we need to deduce the formula for velocity here:
$$v=\frac{dx}{dt}=(10\cos t)'=-10\sin t$$
- For $t=0$: $$v=-10\sin0=-10\times0=0(cm/s)$$
- For $t=\pi/3$: $$v=-10\sin\frac{\pi}{3}=-10\times\frac{\sqrt3}{2}=-5\sqrt3(cm/s)$$
- For $t=3\pi/4$: $$v=-10\sin\frac{3\pi}{4}=-10\times\frac{\sqrt2}{2}=-5\sqrt2(cm/s)$$