A/ 36X2 – 49 = 0B/ (X + 3) (X2 – 3X + 9) – X (X – 1) (X + 1) – 27 = 0 ( 6X)2 – 72 = 0 (0,25Đ)X3 – 3X2 + 9X + 3X2 – 9X + 27 – X (X2 – 1 ) – 27 = 0 (0,25Đ) (6X – 7 ) ( 6X + 7 ) = 0 (0,25Đ)X3 + 27 – X ( X2 – 1 ) – 27 = 0 (0,25Đ) X...
Câu 10 : (2đ) Tìm x biết :
a/ 36x
2
– 49 = 0
b/ (x + 3) (x
2
– 3x + 9) – x (x – 1) (x + 1) – 27 = 0
( 6x)
2
– 7
2
= 0 (0,25đ)
x
3
– 3x
2
+ 9x + 3x
2
– 9x + 27 – x (x
2
– 1 ) – 27 = 0 (0,25đ)
(6x – 7 ) ( 6x + 7 ) = 0 (0,25đ)
x
3
+ 27 – x ( x
2
– 1 ) – 27 = 0 (0,25đ)
x
3
– x
3
+ x = 0 (0,25đ)
Ta có 6x – 7 = 0
7
(0,25đ)
x = 0 (0,25đ)
x
6
Hoặc 6x + 7 = 0
7
(0,25đ)