1. จงหาผลคูณ
1) \(\mathsf{\frac{1}{7} \times \frac{7}{9}}\)
วิธีทำ
\(\mathsf{\frac{1}{7} \times \frac{7}{9}}\) = \(\mathsf{\frac{1}{7_1\mkern-17mu\color{Red}/} \times \frac{7^1\mkern-17mu\color{Red}/}{9}}\)
= \(\mathsf{\frac{1}{1} \times \frac{1}{9}}\)
= \(\mathsf{\frac{1}{9}}\)
ตอบ \(\mathsf{\frac{1}{9}}\)
2) \(\mathsf{(-\frac{2}{3}) \times \frac{1}{5}}\)
วิธีทำ
\(\mathsf{(-\frac{2}{3}) \times \frac{1}{5}}\) = \(\mathsf{\frac{(-2) \times 1}{3 \times 5}}\)
= \(\mathsf{-\frac{2}{15}}\)
ตอบ \(\mathsf{-\frac{2}{15}}\)
3) \(\mathsf{\frac{2}{5} \times 0}\)
ตอบ 0
4) \(\mathsf{(\frac{25}{4})(-\frac{8}{11})}\)
วิธีทำ
\(\mathsf{(\frac{25}{4})(-\frac{8}{11})}\) = \(\mathsf{(\frac{25}{4_1\mkern-14mu\color{Red}/})(-\frac{8^2\mkern-14mu\color{Red}/}{11})}\)
= \(\mathsf{(\frac{25}{1})(-\frac{2}{11})}\)
= \(\mathsf{\frac{(25)(-2)}{(1)(11)}}\)
= \(\mathsf{-\frac{50}{11}}\)
= \(\mathsf{-4\frac{6}{11}}\)
ตอบ \(\mathsf{-4\frac{6}{11}}\)
5) \(\mathsf{(-3\frac{3}{5}) \times \frac{5}{18}}\)
วิธีทำ
\(\mathsf{(-3\frac{3}{5}) \times \frac{5}{18}}\) = \(\mathsf{(-\frac{18}{5}) \times \frac{5}{18}}\)
= \(\mathsf{(-\frac{18^1\mkern-16.5mu\color{Red}/}{5_1\mkern-14mu\color{Red}/}) \times \frac{5^1\mkern-14mu\color{Red}/}{18_1\mkern-16.5mu\color{Red}/}}\)
= \(\mathsf{(-\frac{1}{1}) \times \frac{1}{1}}\)
= (-1) \(\times\) 1
= -1
ตอบ -1
6) \(\mathsf{(-4\frac{8}{11}) \times (-\frac{3}{13})}\)
วิธีทำ
\(\mathsf{(-4\frac{8}{11}) \times (-\frac{3}{13})}\) = \(\mathsf{(-\frac{52}{11}) \times (-\frac{3}{13})}\)
= \(\mathsf{(-\frac{52^4\mkern-18mu\color{Red}/}{11}) \times (-\frac{3}{13_1\mkern-16mu\color{Red}/})}\)
= \(\mathsf{(-\frac{4}{11}) \times (-\frac{3}{1})}\)
= \(\mathsf{\frac{(-4) \times (-3)}{11 \times 1}}\)
= \(\mathsf{\frac{12}{11}}\)
= \(\mathsf{1\frac{1}{11}}\)
ตอบ \(\mathsf{1\frac{1}{11}}\)
7) \(\mathsf{(2\frac{1}{7})(-3\frac{2}{5})}\)
วิธีทำ
\(\mathsf{(2\frac{1}{7})(-3\frac{2}{5})}\) = \(\mathsf{(\frac{15}{7})(-\frac{17}{5})}\)
= \(\mathsf{(\frac{15^3\mkern-18mu\color{Red}/}{7})(-\frac{17}{5_1\mkern-15mu\color{Red}/})}\)
= \(\mathsf{(\frac{3}{7})(-\frac{17}{1})}\)
= \(\mathsf{\frac{(3)(-17)}{(7)(1)}}\)
= \(\mathsf{-\frac{51}{7}}\)
= \(\mathsf{-7\frac{2}{7}}\)
ตอบ \(\mathsf{-7\frac{2}{7}}\)
8) \(\mathsf{(-1\frac{1}{5}) \times \frac{35}{12}}\)
วิธีทำ
\(\mathsf{(-1\frac{1}{5}) \times \frac{35}{12}}\) = \(\mathsf{(-\frac{6}{5}) \times \frac{35}{12}}\)
= \(\mathsf{(-\frac{6^1\mkern-15mu\color{Red}/}{5_1\mkern-15mu\color{Red}/}) \times \frac{35^7\mkern-18mu\color{Red}/}{12_2\mkern-17mu\color{Red}/}}\)
= \(\mathsf{(-\frac{1}{1}) \times \frac{7}{2}}\)
= \(\mathsf{\frac{(-1) \times 7}{1 \times 2}}\)
= \(\mathsf{-\frac{7}{2}}\)
= \(\mathsf{-3\frac{1}{2}}\)
ตอบ \(\mathsf{-3\frac{1}{2}}\)
9) \(\mathsf{(-2\frac{2}{7}) \times \frac{3}{4}}\)
วิธีทำ
\(\mathsf{(-2\frac{2}{7}) \times \frac{3}{4}}\) = \(\mathsf{(-\frac{16}{7}) \times \frac{3}{4}}\)
= \(\mathsf{(-\frac{16^4\mkern-18mu\color{Red}/}{7}) \times \frac{3}{4_1\mkern-14.5mu\color{Red}/}}\)
= \(\mathsf{(-\frac{4}{7}) \times \frac{3}{1}}\)
= \(\mathsf{\frac{(-4) \times 3}{7 \times 1}}\)
= \(\mathsf{-\frac{12}{7}}\)
= \(\mathsf{-1\frac{5}{7}}\)
ตอบ \(\mathsf{-1\frac{5}{7}}\)
10) \(\mathsf{(-\frac{7}{8})(-2\frac{2}{3})}\)
วิธีทำ
\(\mathsf{(-\frac{7}{8})(-2\frac{2}{3})}\) = \(\mathsf{(-\frac{7}{8})(-\frac{8}{3})}\)
= \(\mathsf{(-\frac{7}{8_1\mkern-14.5mu\color{Red}/})(-\frac{8^1\mkern-15mu\color{Red}/}{3})}\)
= \(\mathsf{(-\frac{7}{1})(-\frac{1}{3})}\)
= \(\mathsf{\frac{(-7)(-1)}{(1)(3)}}\)
= \(\mathsf{\frac{7}{3}}\)
= \(\mathsf{2\frac{1}{3}}\)
ตอบ \(\mathsf{2\frac{1}{3}}\)
11) \(\mathsf{3\frac{3}{7} \times 0 \times (-20\frac{5}{9})}\)
ตอบ 0
12) \(\mathsf{3\frac{1}{4} \times (-2) \times 1}\)
วิธีทำ
\(\mathsf{3\frac{1}{4} \times (-2) \times 1}\) = \(\mathsf{\frac{13}{4} \times (-2)}\)
= \(\mathsf{\frac{13}{4_2\mkern-14.5mu\color{Red}/} \times (-2^1\mkern-18mu\color{Red}/\;)}\)
= \(\mathsf{\frac{13}{2} \times (-1)}\)
= \(\mathsf{-\frac{13}{2}}\)
= \(\mathsf{-6\frac{1}{2}}\)
ตอบ \(\mathsf{-6\frac{1}{2}}\)