Geometric Progression

[anthropos]

The first two terms of a geometric progression are x and y where -x < y < x (<--what does this mean?) If the sum of the first n terms is equal to the sum to infinity of the remaining termsm prove that x^n = 2y^n.

 

So I did this.

 

T1 = x = a

T2= y

r= y/x

Tn+1 = y^n

...

x(1-(y/x)^n) = y^n (after substituting a and r values into G.P formulae)

But i don get the answer! what's the prob?