https://vn.answers.yahoo.com/question/index?qid=20100907070553AAEawEd
lời giải rõ ràng rồi đó bạn ^_~
\(\frac{15-\frac{15}{7}-\frac{15}{12}}{3-\frac{3}{7}-\frac{3}{12}}\)
\(=\frac{15\left(1-\frac{1}{7}-\frac{1}{12}\right)}{3\left(1-\frac{1}{7}-\frac{1}{12}\right)}\)
\(=\frac{15}{3}\)
K nha