1/ $\dfrac35+\dfrac47=\dfrac{3×7}{5×7}+\dfrac{4×5}{7×5}=\dfrac{21}{35}+\dfrac{20}{35}=\dfrac{41}{35}$
Mẫu số chung: $15$
$\dfrac4{15}+\dfrac45=\dfrac4{15}+\dfrac{4\times3}{5\times3}=\dfrac4{15}+\dfrac{12}{15}=\dfrac{16}{15}$
$\dfrac56+\dfrac38=\dfrac{5\times8}{6\times8}+\dfrac{3\times6}{8\times6}=\dfrac{40}{48}+\dfrac{18}{48}=\dfrac{58}{48}=\dfrac{58:2}{48:2}=\dfrac{29}{24}$
2/ $\dfrac25+\dfrac3{15}=\dfrac25+\dfrac15=\dfrac35$
$\dfrac{12}{18}+\dfrac56=\dfrac{12}{18}+\dfrac{15}{18}=\dfrac{27}{18}$
