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why internal resistance of voltage source is considered as zero and internal resistance of current source is infinity?


You are missing the words "A perfect" on voltage source and current source. For any real voltage source, taking more and more current from it MUST reduce the output voltage at some point, or you are violating the laws of physics. Likewise, for any constant current source loading more and more onto it HAS to cause the output current to fall as the load increases.

+1

The perfect component is only to help in math analysis. If you continue to add branches to your electrical engineering tree of knowledge you will see that a series resistance is added to the perfect voltage source to more closely resemble the actual source as you learn your lessons.