## Calculus (3rd Edition)

$$1$$
$$(i+j)\cdot (j+k)=i\cdot j+i\cdot k+j\cdot j+j\cdot k\\ =0+0+1+0=1.$$ Note that the product of $i\cdot j=i\cdot k=j\cdot k=0$, because they are perpendicular ($\theta=90^{\circ}$).