mjk sửa lại
a)100x^2 -( x^2+25)^2
=[10x-(x2+25)][10x+(x2+25)]
=(10x-x2-25)(10x+x2+25)
=-(x2-10x+25)(x+5)2
=-(x-5)2(x+5)2
b)(x+4)^3 - 64
=(x+4)3-43
=(x+4-4)[(x+4)2+(x+4).4+16]
=x(x2+8x+16+4x+16+16)
=x(x2+12x+48)
c) x^6 + y^6
=(x2)3+(y2)3
=(x2+y2)(x4+x2y2+y4)