when i split a polygon use the split tool on the editortoolbar ,how can it show the measure of the smaller one's area?i say it not i finish the split,i mean it like when i creating a polygon press "a" and it can show the polygon
Do you want the polygon features labelled as you use split the polygons?
I hope you are working with Geodatabase feature class (shapefiles do not support automatic updation of Shape_Length/Shape_Area).
Go to the properties of polygon layer in Table of Contents > Labels. Keep the Label field as "Shape_Area". Check the "Label features in this layer" option.
So when you split the features, the updated Shape_Area would be filled as labels.
This was my initial thought as well. However, I don't think it will work in this case if the split is not completed.
The only way to do this is to set up labelling as Jayanta suggests, start an edit session, split the polygon, check the area and undo the split if required.