Activating groups are ortho-/para-directing ,while the deactivating groups are meta-directing-why ?

Explanation

The activating groups by exerting their electron-donating +I and /or +R effects increase electorn densities at the ortho-and para-positions to a larger extent than the meta -position. So, the electrophiles `(E^+)` enter preferably the ortho- and para- positions . On the other hand the deactivating groups by exerting their electron withdrawing -I and /or -R effects decrases electron densities of meta-positions to a lesser extent than the ortho -and para-positions . so the electrophile enters preferably the relatively more electron -rich meta- position.