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Note, that not every shower fulfill the obligatory ghusl, to make a proper ghusl(bath) there are three fardh part in it.
1.Passing water into and out of the mouth, i.e. GARGLING.
2. Putting water into the NOSTRILS.
3. Passing water over the entire body.
Note: if any of the obligatory are left out or a HAIRS BREADTH place is left dry the Ghusl will be INCOMPLETE.
When you've done the ghusl with fulfilling the above three points, then there is NO need to do wudhu again, but if you've missed one the above point then it is simple shower and not ghusl, you have to do wudhu before offering salaah.
For more of its understanding, please follow this link
http://www.islamicboard.com/basics-i...ving-wudu.html
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