Bài 11 trang 12 Toán 7 tập 1, Bài 11 tính: a) -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} ight).left( { – {7 over {12}}} ight))

d) (left( { – {3 over {25}}} ight):6)

Lời giải:

a) ({{ – 2} over 7}.{{21} over 8} = {{left( { – 2} ight).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} ight)} over {25.4}} = {{ – 90} over {100}} = {{ – 9} over {10}})

c) (left( { – 2} ight).left( { – {7 over {12}}} ight) = {{left( { – 2} ight)left( { – 7} ight)} over {12}} = {{14} over {12}} = {7 over 6} = 1{1 over 6})

d) (left( { – {3 over {25}}} ight):6 = {{ – 3} over {25}}:{6 over 1} = {{ – 3} over {25}}.{1 over 6} = {{left( { – 3} ight).1} over {25.6}} = {{ – 3} over {150}} = {{ – 1} over {50}})