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1.      \( 7mn+21m-7n-21=7(mn+3m-n-3)=7(m(n+3)-(n+3))= \) [ ];    
   

2.      \( a^3-6a^2-4a+24= \) [ ];    
   

3.      \( abc-9ab+5ac-45a= \) [ ];    
   

4.      \( 8m^2-8n^2-56m^3n+56mn^3= \) [ ];    
   

5.      \( xy^5+y^5-xy^4-y^4= \) [ ].