Bài 11 trang 12 Toán 7 tập 1, Bài 11 tính:2/7 . 21/8; b) 0,24 . 15/4...

Bài 11 Tính

a) ${{ - 2} \over 7}.{{21} \over 8}$

b) $0,24.{{ - 15} \over 4}$

c) $\left( { - 2} \right).\left( { - {7 \over {12}}} \right)$

d) $\left( { - {3 \over {25}}} \right):6$

a) ${{ - 2} \over 7}.{{21} \over 8} = {{\left( { - 2} \right).21} \over {7.8}} = {{ - 42} \over {56}} = {{ - 3} \over 4}$

b) $0,24.{{ - 15} \over 4} = {{24} \over {100}}.{{ - 15} \over 4} = {6 \over {25}}.{{ - 15} \over 4} = {{6.\left( { - 15} \right)} \over {25.4}} = {{ - 90} \over {100}} = {{ - 9} \over {10}}$

c) $\left( { - 2} \right).\left( { - {7 \over {12}}} \right) = {{\left( { - 2} \right)\left( { - 7} \right)} \over {12}} = {{14} \over {12}} = {7 \over 6} = 1{1 \over 6}$

d) $\left( { - {3 \over {25}}} \right):6 = {{ - 3} \over {25}}:{6 \over 1} = {{ - 3} \over {25}}.{1 \over 6} = {{\left( { - 3} \right).1} \over {25.6}} = {{ - 3} \over {150}} = {{ - 1} \over {50}}$