Giải bài 3 trang 167 - SGK Toán lớp 4

a) $\dfrac{2}{9} + x = 1\\\,\,\,\,\,\,\,\,\,\,\,x = 1 - \dfrac{2}{9} \\\,\,\,\,\,\,\,\,\,\,\,x =\dfrac{7}{9}$         

b) $\dfrac{6}{7} - x = \dfrac{2}{3}\\\,\,\,\,\,\,\,\,\,\,\,x = \dfrac{6}{7} - \dfrac{2}{3}\\\,\,\,\,\,\,\,\,\,\,\,x = \dfrac{4}{21}$             

c) $x - \dfrac{1}{2} = \dfrac{1}{4}\\x \,\,\,\,\,\,\,\,\,\,\,=\dfrac{1}{4}+ \dfrac{1}{2}\\x\,\,\,\,\,\,\,\,\,\,\,=\dfrac{3}{4}$