Hi,
to analyse a particular area you must copy this values into new channels (columns). Therefore you can esyly use the panal DIAdem VIEW. You can drag&drop the original channels into a 2D-axis system, define a band cursor (icon on the top), mark the area (icon in the current window) and copy (icon in the current window)it. Now you can use the mathematical function you would like.
Kind regards
Walter Rick