Here, \[P+Q=P-Q\] \[P+Q-P+Q=0\] \[2Q=0\] \[\therefore Q=0\] Thus, the vectors $(P+Q)$ and $(P-Q)$ will be equal if $Q$ is a null vector.
Here, \[P+Q=P-Q\] \[P+Q-P+Q=0\] \[2Q=0\] \[\therefore Q=0\] Thus, the vectors $(P+Q)$ and $(P-Q)$ will be equal if $Q$ is a null vector.