4, 25 - a^2 - 2ab - b^2
= 5^2 - ( a^2 + 2ab + b^2)
= 5^2 - ( a + b)^2
= ( 5 - a - b) ( 5 + a + b)
5, 4x^2 - 25 + ( 2x + 7) ( 5 - 2x)
= ( 2x - 5) ( 2x + 5) + ( 2x + 7) ( 5 - 2x)
= ( 2x - 5) ( 2x + 5) - ( 2x + 7) ( 2x - 5)
= (2x - 5)*[( 2x + 5) - ( 2x + 7)]
= (2x - 5)*( 2x + 5 - 2x - 7 )
= ( 2x - 5)* ( -2)
6, a^2x^2 - a^2y^2 - b^2x^2 + b^2y^2
= a^2(x^2 - y^2) - b^2( x^2 - y^2)
= (a^2 - b^2) ( x^2 - y^2)
= ( a - b) ( a + b) ( x - y) ( x + y)
Bảy Hằng Đẳng Thức:
1,(A + B)^2 = A^2 + 2AB + B^2
2,(A – B)^2 = A^2 – 2AB + B^2
3,A^2 – B^2 = (A – B)(A + B)
4,(A + B)^3 = A^3 + 3A^2B + 3AB^2 + B^3
5,(A – B)^3 = A^3 – 3A^2B + 3AB^2 – B^3
6,A^3 + B^3 = (A + B)(A^2 – AB + B^2)
7,A^3 – B^3 = (A – B)(A^2 + AB + B^2)
