X2 + 4X - Y2 + 4 = ( X2 + 4X + 4) - Y2 = (X+2)2 - Y2 = (X + 2 - Y)...
3) x
2
+ 4x - y
2
+ 4 = ( x
2
+ 4x + 4) - y
2
= (x+2)
2
- y
2
= (x + 2 - y) (x+ 2 +
y)
Ví dụ 2: Tìm x biết:
a) x
3
+ x
2
+ x + 1 = 0 b) x(x - 3) - x + 3 = 0
(x
3
+ x
2
) + (x + 1) = 0 x(x - 3) - (x - 3) = 0
x
2
(x +1) + (x + 1) = 0 (x-1) (x - 3) =
0
( x
2
+1). (x + 1) = 0 Suy ra: x = 1 hoặc x = 3
Suy ra: x = -1 (do x
2