a) [tex]\frac{4}{12} +\frac{1}{2} +\frac{1}{6} =\frac{4}{12} +\frac{6}{12} +\frac{2}{12} =\frac{12}{12} =1[/tex]
b)[tex]\frac{1}{9} +\frac{1}{3} +\frac{5}{9} =\frac{1}{9} +\frac{3}{9} +\frac{5}{9} =\frac{9}{9} =1[/tex]
c)[tex]\frac{1}{18} +\frac{1}{2} +\frac{1}{3} +\frac{2}{3} =\frac{18}{18} =1[/tex]