... −
1+ i
2
2
n +1
1 −
1+ i
2
·
Ta cˆa
`
n t´ınh
1+ i
2
2
n +1
=
1+ i
2
2
2
n
=
i
2
2
n
=
i
2
n
2
2
n
=
1
2
2
n
·
Do d
´o
σ =
1 −
1
2
2
n
1 −
1+ i
2
=
2
1 −
1
2
2
n
1 − i
×
1+ i
1+ i
=
1 ... =
1+
1+ i
2
1+
1+ i
2
2
1+
1+ i
2
2
2
···
1+
1+ i
2
2
n
.
Gia
’
i. Nhˆan v`a chia biˆe
’
uth´u
.
cd
˜achov´o
.
i1−
1+ i...
... i
2
···i
99
· i
10 0
= 1.
2
+
T`ım sˆo
´
nguyˆen n nˆe
´
u:
a) (1 + i)
n
= (1 i)
n
;
b)
1+ i
√
2
n
+
1 − i
√
2
n
=0.
Gia
’
i. 1
+
Ta c´o i
0
=1, i
1
= i, i
2
= 1, i
3
= −i, i
4
=1, i
5
= ... = −3. (1. 4)
T`u
.
(1. 4) ta c´o b = −
3
2a
.Thˆe
´
v`ao (1. 3) ta thu d
u
.
o
.
.
c
4u
2
− 16 u − 9= 0,u= a
2
⇐⇒
u
1
=
8+
√
10 0
4
=
8 +10
4
=
18
4
=
9
2
,
u
2
=
8 −
√...