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 .