As |x| < 1

And y = 1+x+x² +……. (this is an infinite G.P with common ratio x)

∵ |x|<1

∴ using the formula for sum of infinite G.P,we have–

y =

We know that f’(ax+b) = af’(x)

As

But here a = –1 and b = 1

∴