There is another common error that people get into with citing sources.

The have the deplorable habit of citing something that a  "scientist"  writes, but it often turns out

that it is only the scientists opinion, and the book is not a Peer Reviewed Science Publication.

 [Personal opinons are not probative in the sciences.]

  For academic citations,  Wikipedia  would not be  a  "Peer Reviewed"  source.

 However, most of the general information contained on Wikipedia is adequate for  facts.

    In addition,  Wikipedia  draws from many scholarly sources, and although you may want a stronger scholarly or scientific source for your facts than quoting Wikipedia, you can find the quotes in the Wiki pages, observing the "number" which directs you to the book it is quoting, and then you can directly quote the scholary source.  Here is an example:

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"Modern Romance languages are continuations of dialectal forms (vulgar Latin) of the language. Additionally many students, scholars, and some members of the Christian clergy speak it fluently, and it is still taught in some primary, secondary and post-secondary educational institutions around the world.[3][4]"

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    Here is what [3] and [4]   are,   at the bottom of the page, where it shows sources.

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Jump up ^ Hu, Winnie (6 October 2008). "A Dead Language That's Very Much Alive". New York Times. Jump up ^ Eskenazi, Mike (2 December 2000). "The New case for Latin". TIME.

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       So there you have the original source data, which should serve anyone well in any academic or other context.   (It would be better if you had the page number though.)

I did have same experience with you when I was doing my Final Assignment. I was also curious of why Wikipedia couldn't be accepted in academic writing. Eventually I found the reason. Some Wikipedia entries have no author stated clearly, or although it is there, may be just an alias. While in academic writing, we do need a clear writer's name, don't we?

I think you have to be carefully and be secure about which are the main information sources...

yes, i got it. thanks for all the information, Hoan, Fransisco, Afie, Bruce, Comandante, Devin :) Wikipedia is a source to identify the real sources, isn't it?
yes we need the clear name of Author, Afie. And we've gotta find a book or a pdf book which has ISBN number to get quotation.