Maybe stupid question but i am not sure.If I have public address for bitcoin and public address for ethereum,can i know if these public addresses belong to same seed even if I dont have seed?
In a word, No.There is no way to link a BTC address to an ETH address unless you know what the common starting point is. In this case, it would be the BIP32 root key (which is derived from the BIP39 seed).
Go here: https://iancoleman.io/bip39/
Generate a random mnemonic, change the coin from BTC to ETH... ETH to BTC... the only constants are the mnemonic, BIP39 seed and BIP39 root key... all the other data used for private key/public address derivation changes.
this is not by design or by force though. this is by choice. in other words you can technically use one seed and then derive the same exact keys for every coin that supports BIP32 and use those keys. but the "choice" is that tools like Ledger use a different derivation path with a hardened key so that they resulting key pairs will be different.
otherwise the technique is the same, during your second depth key extension, instead of using index 2,147,483,649 (0') you use index 2,147,483,709 (60') and you get ETH keys
