A, 1 3 3+ + + + +2 33
Bài 15: Chứng minh rằng:
a,
1 3 3
+ + + + +
2
3
3
... 3
11
4
b,
5 5
+
2
+ + +
5
3
... 5 30
8
HD:
a, Ta có:
A= + + + + + +1 3 32
33
... 310
311
= + +( ) ( )
1 3 3 1 32
+ + +... 310
( )
3 1+2
4
10
4 3 .4 3 .4 .... 3 .4 4
A
= +
+
+
+
b, Ta có:
B= + + + + + = +5 52
53
54
... 58
(
5 52
) (
+ 53
+54
)
+ +...(
57
+58
)
2
6
30 5 .30 ... 5 .30
B
=
+
+ +