Question

Bài 1:Tính bằng cách thuận tiện nhất A, 4/5 × 7/6 ×5/2 ×6/14 B,13/7 :8/3-5/17:8/3 C,1/6+1/12+1/20+1/30

Answer 1

\(A=\dfrac{4}{5}\times\dfrac{7}{6}\times\dfrac{5}{2}\times\dfrac{6}{14}=\dfrac{2\times2\times7\times5\times2\times8}{5\times2\times3\times2\times2\times7}=\dfrac{8}{3}\)

 

\(B=\dfrac{13}{7}:\dfrac{8}{3}-\dfrac{5}{7}:\dfrac{8}{3}=\left(\dfrac{13}{7}-\dfrac{5}{7}\right):\dfrac{8}{3}=\dfrac{8}{7}:\dfrac{8}{3}=\dfrac{3}{7}\)

 

\(C=\dfrac{1}{6}\times\dfrac{1}{12}\times\dfrac{1}{20}\times\dfrac{1}{30}=\dfrac{1}{2\times3}+\dfrac{1}{3\times4}+\dfrac{1}{4\times5}+\dfrac{1}{5\times6}\)

\(=\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}=\dfrac{1}{2}-\dfrac{1}{6}=\dfrac{1}{3}\)