How to check relays of tx? I believe a tx is made of an output of previous tx.
So, I also believe that all tx are traced back to tx originally created (as a mining reward).
But, in blockchain.info and blockexplorer.com, I could not find relays of tx. 
We can know only connection of addresses in tx page.

Then, I have three questions about relays of tx.
1. Is a id(hash) of tx included in next tx id?
2. Is there a way to get information of previous tx in these websites ?
3. Why do these websites hide connection of tx?  

Thank you for helping me!
Block explorers dumb down information so that it is easier for the common person to read. In reality, Bitcoin is not actually how block explorers display information; addresses do not exist at the protocol level and coins are not sent from addresses to other addresses.


  
  Is a id(hash) of tx included in next tx id?
  


Yes.


  
  Is there a way to get information of previous tx in these websites ?
  


Yes. Most block explorers have an advanced mode that will display this information, or at least link back to the transaction outputs that are being spent. On blockchain.info, you will see a link labeled Show Scripts &amp; Coinbase when you view a transaction, click that and you will see the advanced information about the transaction.


  
  Why do these websites hide connection of tx?
  


Because it isn't useful to most people and is confusing.