Giải câu 2 trang 13 sách phát triển năng lực toán 7 tập 1.
Cách 1:
A = $\left ( 8-\frac{3}{4}+\frac{1}{3} \right ) - \left ( 6+\frac{5}{4}-\frac{4}{3} \right )-\left ( 4-\frac{7}{4}+\frac{5}{3} \right )$
= $(8-6-4) + \left ( \frac{-3}{4}-\frac{5}{4}+\frac{7}{4} \right ) + \left ( \frac{1}{3}+\frac{4}{3}-\frac{5}{3} \right )$
= $-2 +\frac{-1}{4} + 0$
= $\frac{-9}{4}$
B = $\frac{3}{7}.\frac{9}{26}-\frac{1}{14}.\frac{1}{13}$
= $\frac{3.9}{7.26}-\frac{1}{14.13}$
= $\frac{27}{14.13}-\frac{1}{14.13}$
= $\frac{27-1}{14.13}$
= $\frac{26}{14.13}$
= $\frac{2. 13}{2.7.13}$
= $\frac{1}{7}$
C = $\left ( \frac{-3}{4}+\frac{2}{5} \right ):\frac{3}{7} + \left ( \frac{3}{5}+\frac{-1}{4} \right ):\frac{3}{7}$
= $\left ( \frac{-3}{4}+\frac{2}{5} \right ).\frac{7}{3} + \left ( \frac{3}{5}+\frac{-1}{4} \right ).\frac{7}{3}$
= $\left (\frac{-3}{4}+\frac{2}{5}+\frac{3}{5}+\frac{-1}{4} \right ).\frac{7}{3}$
= $\left ( \frac{-3-1}{4}+\frac{2+3}{5} \right ).\frac{7}{3}$
= 0.$\frac{7}{3}$
= 0
Cách 2: Tính trực tiếp.
A = $\left ( 8-\frac{3}{4}+\frac{1}{3} \right ) - \left ( 6+\frac{5}{4}-\frac{4}{3} \right )-\left ( 4-\frac{7}{4}+\frac{5}{3} \right )$
= $\frac{91}{12}-\frac{71}{12}-\frac{47}{12}$
= $\frac{91 - 71 - 47}{12}$
= $\frac{-27}{12}$
= $\frac{-9}{4}$
B = $\frac{3}{7}.\frac{9}{26}-\frac{1}{14}.\frac{1}{13}$
= $\frac{3.9}{7.26}-\frac{1}{14.13}$
= $\frac{27}{182}-\frac{1}{182}$
= $\frac{27-1}{182}$
= $\frac{26}{182}$
= $\frac{1}{7}$
C = $\left ( \frac{-3}{4}+\frac{2}{5} \right ):\frac{3}{7} + \left ( \frac{3}{5}+\frac{-1}{4} \right ):\frac{3}{7}$
= $\frac{-7}{20}.\frac{7}{3} + \frac{7}{20}.\frac{7}{3}$
= $\frac{-49}{60}+\frac{49}{60}$
= 0
b. Dùng máy tính kiểm tra kết quả:
A = $\frac{-9}{4}$
B = $\frac{1}{7}$
C = 0