Câu 86 trang 25 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})