Bài 5: Tìm x, biết:a) $x\times\frac{21}{25}=\frac{-7}{10}$b) $\frac{-3}{2...

Phương pháp giải:
a) $x\times \frac{21}{25} = \frac{-7}{10}$
Ta có: $x=\frac{-7}{10}/\frac{21}{25}=\frac{-7}{10} \times \frac{25}{21}=\frac{-5}{6}$

b) $\frac{-3}{20}x = \frac{5}{12} - \frac{2}{5}$
Ta có: $x=\frac{5}{12} - \frac{2}{5} \div \frac{-3}{20} = \frac{5}{12} - \frac{2}{5} \times \frac{-20}{3} = \frac{5}{12} + \frac{8}{15} = \frac{1}{60} \div \frac{-3}{20} = \frac{-1}{9}$

c) $\frac{2}{9} \div x = \frac{1}{18} \div 0.375$
Ta có: $x=\frac{2}{9} \div \frac{1}{18} \div 0.375 = \frac{2}{9} \div \frac{1}{18} \times \frac{8}{3} = \frac{2}{9} \div \frac{4}{27} = \frac{3}{2}$

d) $(-\frac{11}{12}) \div 2x = \frac{5}{2} + \frac{1}{4}$
Ta có: $2x = (-\frac{11}{12}) \div \frac{11}{4} = (-\frac{11}{12}) \div \frac{11}{4} = -\frac{11}{12} \times \frac{4}{11} = -\frac{1}{3}$
Suy ra: $x = \frac{-1}{6}$

Vậy:
a) $x = \frac{-5}{6}$
b) $x = \frac{-1}{9}$
c) $x = \frac{3}{2}$
d) $x = \frac{-1}{6}$