原式=

3

2

-3+

1

6

+3+

1

12

-5+

1

20

+5+

1

30

-7+

1

42

+7+

1

56

-9+

1

72

+9+

1

90

=2-

1

2

+

1

6

+

1

12

+

1

20

+

1

30

+

1

42

+

1

56

+

1

72

+

1

90

=2-

1

2

+

1

2

-

1

3

+

1

3

-

1

4

+

1

4

-

1

5

+

1

5

-

1

6

+

1

6

-

1

7

+

1

7

-

1

8

+

1

8

-

1

9

+

1

9

-

1

10

=2-

1

10

=

19

10

．