Bài 95 trang 45 SGK Toán 7 tập 1

Đề bài

Tính giá trị biểu thức: 

$A =  - 5,13:\left( {5{5 \over {28}} - 1{8 \over 9}.1,25 + 1{{16} \over {63}}} \right)$

$B = \left( {3{1 \over 3}.1,9 + 19,5:4{1 \over 3}} \right).\left( {{{62} \over {75}} - {4 \over {25}}} \right)$

Hướng dẫn giải

$\eqalign{ & A = - 5,13:\left( {5{5 \over {28}} - 1{8 \over 9}.1,25 + 1{{16} \over {63}}} \right) \cr & = - 5,13:\left( {{{145} \over {28}} - {{17} \over 9}.{{125} \over {100}} + {{79} \over {63}}} \right) \cr & = - 5,13:\left( {{{145} \over {28}} - {{17} \over 9}.{5 \over 4} + {{79} \over {63}}} \right) \cr & = - 5,13:\left( {{{145} \over {28}} - {{85} \over {36}} + {{79} \over {63}}} \right) \cr & = - 5,13:{{57} \over {14}} = - 5,13.{{14} \over {57}} \cr & = {{ - 71,82} \over {57}} = -1,26 \cr}$

$\eqalign{ & B = \left( {3{1 \over 3}.1,9 + 19,5:4{1 \over 3}} \right).\left( {{{62} \over {75}} - {4 \over {25}}} \right) \cr & = \left( {{{10} \over 3}.1,9 + 19,5:{{13} \over 3}} \right).\left( {{{62 - 12} \over {75}}} \right) \cr & = \left( {{{19} \over 3} + {{58,5} \over {13}}} \right).{{50} \over {75}} \cr & = \left( {{{19} \over 3} + 4,5} \right).{2 \over 3} \cr & = {{19 + 13,5} \over 3}.{2 \over 3} \cr & = {{32,5} \over 3}.{2 \over 3} = {{65} \over 9} = 7{2 \over 9} \cr}$