a)3a2- 6ab + 3b2 - 12c2
=(\(\sqrt{3}\)a - \(\sqrt{3}\)b)2 - (\(\sqrt{12}\)c)2
=[(\(\sqrt{3}\)a - \(\sqrt{3}\)b) - \(\sqrt{12}\)c ) x (\(\sqrt{3}\)a - \(\sqrt{3}\)b) + \(\sqrt{12}\)c]
b) x2 - 25 + y2 + 2xy
=(x + y)2 - 52
=[(x + y) - 5] x [(x + y) + 5]
c) x2y - x3 - 9y +9x
= y(x2-9) -x(x2-9)
= (x2-9) x (y - x)