①4x-7×1.3=9.9;②1 1 2 :0.4=1.35:x;③(2+5+8+…+197)÷(4+7+10+…+19

1个回答

  • ①4x-7×1.3=9.9,

    4x-9.1=9.9,

    4x-9.1+9.1=9.9+9.1,

    4x=19,

    4x÷4=19÷4,

    x=4

    3

    4 ;

    ②1

    1

    2 :0.4=1.35:x;

    1

    1

    2 x=0.4×1.35,

    3

    2 x=0.54,

    3

    2 x×

    2

    3 =0.54×

    2

    3 ,

    x=0.36;

    ③(2+5+8+…+197)÷(4+7+10+…+199),

    =[(2+197)×65÷2]÷[(4+199)×65÷2],

    =[199×65÷2]÷[203×65÷2],

    =

    199×65×

    1

    2

    203×65×

    1

    2 ,

    =

    199

    203 ;

    ④设1-

    1

    2 -

    1

    3 -…-

    1

    2010 =a,

    1

    2 +

    1

    3 +…+

    1

    2010 =b,则a+b=1.

    (1-

    1

    2 -

    1

    3 -…-

    1

    2010 )×(

    1

    2 +

    1

    3 +…+

    1

    2011 )-(1-

    1

    2 -

    1

    3 -…-

    1

    2011 )×(

    1

    2 +

    1

    3 +…+

    1

    2010 ),

    =a×(b+

    1

    2011 )-(a-

    1

    2011 )×b,

    =ab+

    1

    2011 ×a-ab+

    1

    2011 ×b,

    =

    1

    2011 ×(a+b),

    =

    1

    2011 .