1)3/m+ 5/n=3n/mn+5m/nm=(3n+5m)/nm
2) 4/x- 3/xy=4y/xy-3/xy=(4y-3)/xy
3) 7/9ab-13/12ab=(7*4)/(9*4ab)=(13*3)/(12*3ab)=(28-39)/36ab=-11/36ab
1)3/m+ 5/n=3n/mn+5m/nm=(3n+5m)/nm
2) 4/x- 3/xy=4y/xy-3/xy=(4y-3)/xy
3) 7/9ab-13/12ab=(7*4)/(9*4ab)=(13*3)/(12*3ab)=(28-39)/36ab=-11/36ab