A + B = - 1 , AB = - 12 = 2
209. Ta cĩ : a + b = - 1 , ab = -
1
2
=
2
.
4
nên : a
a
4
+ b
4
= (a
2
+ b
2
)
2
– 2a
2
b
2
=
9 1 17
4 9
− =
8
; a
3
+ b
3
= (a + b)
3
– 3ab(a + b) = - 1 -
3
7
4
= −
4
Do đĩ : a
7
+ b
7
= (a
3
+ b
3
)(a
4
+ b
4
) – a
3
b
3
(a + b) =
−
7 17
4 8
.
− −
64
1
÷
( )
− = −
1
239
64
.