## Thomas' Calculus 13th Edition

$$t=\dfrac{\pi}{3}$$
Our aim is to find the value of $t$. $(2,4 \sin (t/2), 3-\dfrac{t}{\pi} ) \cdot (1,-1,0) =0$ This implies that $2-4 \sin (t/2)=0$ $\implies 2=-4 \sin (t/2)$ $\implies t=\dfrac{\pi}{3}$