Salesforce concept : Limitation of Rollup Summary and Validation Rule

Question : Rollup summary CANNOT be used in formula field ?
Answer : No .
Rollup Summary can be used to group field values of child record .

Even thought it is formula type field , however ,
grouped field values CANNOT be cross-object formula field.

Question : How to validate the field values changed by Approval process ?
Answer : Validation is NOT a choice . Since field update fired by approval process will NOT initiate validation rule. (More explicit answer from google  , Validation Rules fire before workflow rules, so the record has to meet the validation rule criteria before the workflows get a chance to run. So, the validation rule will not work when you update the workflow field.)

Tigger can be used to validate the field values changed by approval process.