How did Roots and The Cosby Show challenge previous sterotypical representations of black people in TV dramas
Roots - For the first time on U.S. television some of the realities of slavery--brutality, rape, enforced de-culturation--were confronted over a protracted period, and through individual characters with whom, as they fought to escape or survive, the audience could identify. There was emphasis on the centuries and decades before the 1970s, which the ahistorical vector in U.S. culture easily cushions from application to the often devastating here and now. Nonetheless, it was a signal achievement. This shows more realistic views of how it actually happened. 
The Cosby Show - The show attracted a certain volume of hostile comment, some of it smugly supercilious. The fact it was popular with white audiences in the South, and in South Africa, was a favorite quick shot to try to debunk it. Some critics claimed it fed the mirage that racial injustice could be overcome through individual economic advance, others that it primly fostered Reaganite conservative family values. Both were indeed easily possible readings of the show within contemporary U.S. culture. Yet critics often seemed to think a TV text could actually present a single monolithic meaningfulness or set up a firewall against inappropriate readings. The black man was a doctor and was shown in a positive way. An educated person who was in a profession in a wealthy area. His children went to a private area. 


The black people were non-existant they didn't really talk, and they were usually slaves or criminals in the shows. They were represented in a bad way.


What does the article say about the represntaion of black policman in programmes such as Miami Vice and Hawaii Five-O?
Historically the side-kick of the main person is of a different ethnicity and is often shown as the person who was not in charge or would be the comic character of the programme.