What is the difference between these two sentences?
A: The white house which Mr Trump lives in is beautiful.
B: The white house in which Mr Trump lives is beautiful.
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Assuming you use each of these noun phrases in a sentence like "The white House in which Mr. Trump lives is beautiful" vs. "The white House (which) Mr. Trump lives in is beautiful", they are functionally equivalent with no difference in meaning.
However, you will sometimes have people point out the latter as "wrong", especially in writing. This is because of a rule, taught in many English classes, claiming you shouldn't end sentences (or, more accurately, clauses) with a preposition. While this rule is not a part of native English grammar (you'll hear it "broken" all the time), it was invented a ways back by those who wanted English to be more like Latin and has become one of those rules people will often insist you follow in formal writing, despite it not being part of typical speech. So in a formal essay you're more likely to see "The white house in which Mr. Trump lives" but in spoken English you're more likely to see "The white house Mr. Trump lives in".