Letter can be used for Greet. Greet is related to Introduce. Introduce is the opposite of Conclusion. 
Tax is related to Fine. Fine is a synonym of Beautiful. Mathematics can be Beautiful. 
Letter --UsedFor--> Greet <--RelatedTo--> Introduce <--Antonym--> Conclusion 
Tax <--RelatedTo--> Fine <--Synonym--> Beautiful <--HasProperty-- Mathematics 