a)[tex] \frac{5}{4} + \frac{7}{4} = \frac{12}{4} [/tex]⇔[tex] \frac{12}{4} [/tex] + 1=[tex] \frac{12+4}{4} [/tex] = [tex] \frac{16}{4} [/tex] = 4
b) [tex] \frac{11}{9} - \frac{2}{9} = \frac{9}{9} [/tex]⇔[tex] \frac{9}{9} - 1= \frac{9-9}{9} = \frac{0}{9} [/tex]