Câu 86 trang 25 Sách Bài Tập (SBT) Toán Lớp 6 tập 2

Tính.

Tính:

a) ({ m{}}{2 over 3} + {1 over 5}.{{10} over 7})

b) ({7 over {12}} - {{27} over 1}.{1 over {18}})

c) (left( {{{23} over {41}} - {{15} over {82}}} ight).{{41} over {25}})

d) ({ m{}}left( {{4 over 5} + {1 over 2}} ight).left( {{3 over {13}} - {8 over {13}}} ight))

Giải

a) ({ m{}}{2 over 3} + {1 over 5}.{{10} over 7} )

(= {2 over 3} + {{1.10} over {5.7}} )

(= {2 over 3} + {2 over 7} )

(= {{14} over {21}} + {6 over {21}})

(= {{20} over {21}}) 

b) ({7 over {12}} - {{27} over 1}.{1 over {18}} )

(= {7 over 2} - {{27.1} over {7.18}} )

(= {7 over 2} - {3 over {14}} )

(= {{49} over {14}} + {{ - 3} over {14}} )

(= {{46} over {14}} = {{23} over 7}) 

c) (left( {{{23} over {41}} - {{15} over {82}}} ight).{{41} over {25}} )

(= left( {{{46} over {82}} + {{ - 15} over {82}}} ight).{{41} over {25}} )

(= {{31} over {82}}.{{41} over {25}} )

(= {{31.41} over {82.25}} = {{31} over {50}}) 

d) ({ m{}}left( {{4 over 5} + {1 over 2}} ight).left( {{3 over {13}} - {8 over {13}}} ight) )

(= left( {{8 over {10}} + {5 over {10}}} ight).left( {{3 over {13}} + {{ - 8} over {13}}} ight))

(= {{13} over {10}}.{{ - 5} over {13}} = {{13.( - 5)} over {10.13}} = {{ - 1} over 2}) 

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