Factorise:
25a2 - 4b2 + 28bc - 49c2
We have -4b2 + 28bc - 49c2 = (-1)[4b2 - 28bc + 49c2]
= -1[(2b)2 - 2(2b)(7c) + (7c)2]
= - [2b - 7c]2
∴ 25a2 - 4b2 + 28bc - 49c2 = 25a2 - (2b - 7c)2
Now using a2 -b2 = (a + b) (a - b), we have
[5a]2 - [2b - 7c]2 = (5a + 2b - 7c)(5a- 2b + 7c)
∴ 25a2 -4a2 + 28bc - 49c2 = (5a + 2b - 7c)(5a - 2b + 7c)