Bài 90 trang 45 SGK Toán 7 tập 1

Đề bài

Thực hiện các phép tính :

a) \(\left ( \dfrac{9}{25} -2. 18\right ):\left ( 3\dfrac{4}{5} +0,2\right );\)

b) \(\dfrac{5}{18}-1,456:\dfrac{7}{25}+4,5\cdot \dfrac{4}{5}.\)

Lời giải

a) \(\left ( \dfrac{9}{25} -2\cdot 18\right ):\left ( 3\dfrac{4}{5} +0,2\right )\)

\(=\left ( \dfrac{9}{25} -2\cdot 18\right ):\left ( \dfrac{19}{5} +0,2\right )\)

\(=(0,36-36):(3,8+0,2)\)

\(=-35,64:4=-8,91.\)

b) \(\dfrac{5}{18}-1,456:\dfrac{7}{25}+4,5\cdot \dfrac{4}{5}\)

\(=\dfrac{5}{18}-1,456\cdot \dfrac{25}{7}+(4,5:5).4\)

\(=\dfrac{5}{18}-(1,456:7). 25+0,9. 4\)

\(=\dfrac{5}{18}-0,208. 25+3,6\)

\(=\dfrac{5}{18}-5,2+3,6\)

\(=\dfrac{5}{18}+(-5,2+3,6)\)

\(=\dfrac{5}{18}-1,6=\dfrac{5}{18}-\dfrac{8}{5}\)

\( = \dfrac{{25}}{{90}} - \dfrac{{144}}{{90}}\) \(=\dfrac{-119}{90}=-1\dfrac{29}{90}.\)


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