脱式计算下面各题(能简算的要简算)①( 11 12 - 3 4 )÷( 2- 1 2

脱式计算下面各题(能简算的要简算)①( 11 12 - 3 4 )÷( 2- 1 2 )② 1 1×2 + 1 2×3 + 1 3×4 + 1 4×5 + 1 5×6 + 1 7×8 ③( 5 7 - 4 21 )÷ 5 21 ④30.8÷[14-(9.85+1.07)].

①(
11
12
-
3
4
)÷( 2-
1
2
),
=(
11
12
-
9
12
)÷
3
2

=
1
6
×
2
3

=
1
9

1
1×2
+
1
2×3
+
1
3×4
+
1
4×5
+
1
5×6
+
1
7×8

=(1-
1
2
)+(
1
2
-
1
3
)+(
1
3
-
1
4
)+(
1
4
-
1
5
)+(
1
5
-
1
6
)+(
1
7
-
1
8
),
=1 -
1
2
+
1
2
-
1
3
+
1
3
-
1
4
+
1
4
-
1
5
+
1
5
-
1
6
+
1
7
-
1
8

=1-
1
6
+
1
7
-
1
8

=
5
6
+
1
56

=
143
168

③(
5
7
-
4
21
)÷
5
21

=(
15
21
-
4
21
)×
21
5

=
11
21
×
21
5

=
11
5

=2
1
5

④30.8÷[14-(9.85+1.07)],
=30.8÷(14-10.92),
=30.8÷3.08,
=10.
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