To understand the relationship between CoreOs and Kubernetes you can have a look at the following Kubernetes video further you can read my answer where I have put my point to make you understand it easily:-
Where CoreOS is present and used:-
Now coming to Kubernetes, It is a platform for running containers and potentially other workloads across multiple machines such that you are less concerned with where they run than formerly. Kubernetes was created by Google but now it is an open-source technology which is potentially in competition with other products, such as CoreOS’s fleet and Docker’s Swarm.
Moreover this you can run Kubernetes on CoreOS if you wish. To complicate things, Google Ventures invested in CoreOS, but this decision was made independently of the decision to open-source Kubernetes. If you are more into reading then you can have a look at the following DevOps tutorial to understand these concepts in a much better way. If these things fascinate you and you want to make your career in this field then you must have to have a certificate in it. You can take up the following Kubernetes training Course.
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