what is bitcoin mining actually doing ? 
Mining is a process to determine the canonical order in which transactions happen. It is also a way to introduce money into the system since without central issuer who would be in charge of issuing money and how would they do it fairly?In other words miners build block chains , they get a reward for each new valid block they create.
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Bitcoin mining is a process of adding transaction records to bitcoin's public ledger of the past transaction or known as blockchain. This blockchain serves to approve transactions to the rest of the network as having taken place. This bitcoin nodes use the blockchain to distinguish legitimate bitcoin transactions from attempts to re spend coins that have already been spent elsewhere. Miners are then awarded with fees that is paid by the users sending transactions.