แบบฝึกหัด 4.6 ก ข้อ 3 [ม.1 เล่ม 1 บทที่ 4 ทศนิยมและเศษส่วน : สสวท. 2560]

1) \(\mathsf{[(-\frac{3}{10}) \times \frac{7}{15}] + [(-\frac{3}{10}) \times \frac{8}{15}]}\)

 
= \(\mathsf{(-\frac{3}{10}) \times \frac{15}{15}}\)
 
= \(\mathsf{(-\frac{3}{10}) \times 1}\)
 
= \(\mathsf{-\frac{3}{10}}\)

 
2) \(\mathsf{[\frac{11}{9} \times (-3)] + [(-3) \times \frac{7}{9}]}\)

 
= \(\mathsf{(\frac{11}{9} + \frac{7}{9}) \times (-3)}\)
 
= \(\mathsf{\frac{18}{9} \times (-3)}\)
 
= \(\mathsf{\frac{18^2\mkern-18mu\color{Red}/}{9_1\mkern-15mu\color{Red}/} \times (-3)}\)
 
= \(\mathsf{2 \times (-3)}\)
 
= 6

 
3) \(\mathsf{[\frac{3}{7} \times (-\frac{5}{2})] \; – \, (\frac{3}{7} \times \frac{25}{4})}\)

 
= \(\mathsf{\frac{3}{7} \times [(-\frac{5}{2} \times \frac{2}{2}) + (-\frac{25}{4})]}\)
 
= \(\mathsf{\frac{3}{7} \times [(-\frac{10}{4}) + (-\frac{25}{4})]}\)
 
= \(\mathsf{\frac{3}{7} \times (-\frac{35}{4})}\)
 
= \(\mathsf{\frac{3}{7_1\mkern-17mu\color{Red}/} \times (-\frac{35^5\mkern-18mu\color{Red}/}{4})}\)
 
= \(\mathsf{\frac{3}{1} \times (-\frac{5}{4})}\)
 
= \(\mathsf{-\frac{15}{4}}\)
 
= \(\mathsf{-3\frac{3}{4}}\)

 
4) \(\mathsf{[\frac{3}{4}(-\frac{4}{5})] \; – \, [\frac{2}{5}(-\frac{4}{5})]}\)

 
= \(\mathsf{[(\frac{3}{4} \times \frac{5}{5}) \; – \, (\frac{2}{5} \times \frac{4}{4})](-\frac{4}{5})}\)
 
= \(\mathsf{(\frac{15}{20} \; – \, \frac{8}{20})(-\frac{4}{5})}\)
 
= \(\mathsf{(\frac{7}{20})(-\frac{4}{5})}\)
 
= \(\mathsf{(\frac{7}{20_5\mkern-18mu\color{Red}/})(-\frac{4^1\mkern-15mu\color{Red}/}{5})}\)
 
= \(\mathsf{(\frac{7}{5})(-\frac{1}{5})}\)
 
= \(\mathsf{-\frac{7}{25}}\)