## Trigonometry 7th Edition

$(1,1)\cdot(1,-1)=0$
If the vectors are perpendicular, their dot product must be zero. Dot product$=(1,1)\cdot(1,-1)=1(1)+1(-1)=1-1=0$. Therefore, $\textbf {i}+\textbf {j}$ and $\textbf {i}-\textbf {j}$ are perpendicular.