Giải bài 95 trang 45 SGK Toán 7 Tập 1
Tính giá trị các biểu thức:
\(A = -5,13 : \left(5\dfrac{5}{28} - 1\dfrac{8}{9}.1,25 + 1\dfrac{16}{63}\right)\)
\(B = \left(3\dfrac{1}{3}.1,9 + 19,5 : 4\dfrac{1}{3}\right) . \left(\dfrac{62}{75} - \dfrac{4}{25}\right)\)
\(\begin{align*}A &= -5,13 : \left(5\dfrac{5}{28} - 1\dfrac{8}{9}.1,25 + 1\dfrac{16}{63}\right) \\&= -5,13 : \left(\dfrac{145}{28} - \dfrac{17}{9}.\dfrac{125}{100} + \dfrac{79}{63}\right) \\&= -5,13 : \left(\dfrac{145}{28} - \dfrac{17}{9}.\dfrac{5}{4} + \dfrac{79}{63}\right)\\&= -5,13 : \left(\dfrac{145}{28} - \dfrac{85}{36} + \dfrac{79}{63}\right) \\&= -5,13 : \left(\dfrac{1305}{252} - \dfrac{595}{252} + \dfrac{316}{252}\right)\\&=-5,13 : \dfrac{1026}{252}\\&= -5,13 : \dfrac{57}{14}\\&= -5,13.\dfrac{14}{57} \\&= \dfrac{-71,82}{57 } = -1,26 \end{align*} \)
\(\begin{align*}B &= \left(3\dfrac{1}{3}.1,9 + 19,5 : 4\dfrac{1}{3}\right) . \left(\dfrac{62}{75} - \dfrac{4}{25}\right) \\&= \left(\dfrac{10}{3}.1,9 + \dfrac{19}{5}:\dfrac{13}{3} \right).\left(\dfrac{62}{75} - \dfrac{12}{75}\right) \\&=\left(\dfrac{19}{3} + \dfrac{19}{5}.\dfrac{3}{13} \right).\dfrac{50}{75} \\&= \left(\dfrac{19}{3} + \dfrac{58,15}{13} \right).\dfrac{2}{3} \\&= \left(\dfrac{19}{3} + 4,5 \right).\dfrac{2}{3} \\&=\dfrac{19 + 13,5}{3} .\dfrac{2}{3} \\&=\dfrac{32,5}{3} .\dfrac{2}{3} \\&= \dfrac{65}{9} \\&= 7\dfrac{2}{9}\end{align*}\)
Lưu ý: Khi biểu thức có chưa ngoặc thì ta thực hiện tính trong ngoặc trước.