(1) 1+4
1
6 +7
1
12 +10
1
20 +13
1
30 +16
1
42 +19
1
56 +22
1
72 +25
1
90 ,
=(1+4+7+10+13+16+19+22+25)+(
1
6 +
1
12 +
1
20 +
1
30 +
1
42 +
1
56 +
1
72 +
1
90 ),
=117+(
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 ),
=117+(
1
2 -
1
10 ),
=117+
2
5 ,
=117
2
5 ;
(2)设
17
11 +
18
21 +
19
31 +
20
41 =a,
18
21 +
19
31 +
20
41 =b,则a-b=
17
11 +
18
21 +
19
31 +
20
41 -(
18
21 +
19
31 +
20
41 )=
17
11 ,那么:
(
17
11 +
18
21 +
19
31 +
20
41 )×(
18
21 +
19
31 +
20
41 +
21
51 )-(
17
11 +
18
21 +
19
31 +
20
41 +
21
51 )×(
18
21 +
19
31 +
20
41 ) ,
=a×(b+
21
51 )-(a+
21
51 )×b,
=ab+
21
51 a-ab-
21
51 b,
=
21
51 ×(a-b),
=
21
51 ×
17
11 ,
=
7
11 .