energy of oscillation

Take initial extension to be x(kx=100) and extension after pulling it be y.
Now energy of oscillation = extra energy stored in it after pulling.
=1/2*k*(y²-x²)
=1/2*k(y-x)(y+x)

Now using pythagoras y-x=.0002
Now approximating (y+x) to 2x (will not work if k is too large as the difference will be magnified)

=1/2*k*.0002*2x
=.02=1/50

thnx swaraj.