Hello Hanuman My dictionaries say both are correct, but actually "watched it on TV" is much more idiomatic than "watched it on the TV" (in Google, 40,000 hits vs 825 hits). As for the radio, my dictionaries also say that "hear/listen to it on the radio" is more idiomatic than "hear/listen to it on radio". Actually, in Google, "heard it on the radio" hits 169,000 pages whereas "heard it on radio" does only 930 pages.
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