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Method 1:
\Factor the each monomial.
\18 = 2 · 3 · 3
\45mn = 3 · 3 · 5 · m · n
\The GCF is the product of the common prime factors in each monomial.
\GCF = 2 · 3 = 6 \ \ The greatest common factor of 18 and 45mn is 6.
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The greatest common factor of 18 and 45mn is 6.