Question

tính:  \(3,5.\frac{4}{49}-\left[2,\left(4\right).2,\left(45\right)\right]:\left(-8,4\right)\)

Answer 1

ĐẶt A = 2,(4)

 10A  = 24 + 0,(4)

10 A - A = 24 + 0,(4) - 2,(4) = 24 + 0,(4) - 2 - 0,(4) = 22

9A   =  22

=> A = 22/9

Đặt B = 2,(45)

100b = 245,(45)

100b-  b = 245,(45) - 2,(45)

 99b      = 243

b           = 243 /99

             = 27/11

Thay vào bt ta có

\(3,5\cdot\frac{4}{49}-\left[\frac{22}{9}\cdot\frac{27}{11}\right]:\left(-8,4\right)\)

\(=\frac{7}{2}\cdot\frac{4}{49}-6:-\frac{42}{5}\)

\(=\frac{2}{7}-6\cdot-\frac{5}{42}=\frac{2}{7}+\frac{5}{7}=1\)