How to use the element method to prove the following sets are equal?

I have been asked to describe the following sets, and then prove my answers using the element method, but i am not sure how to do this.

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I am trying to prove that (b) is equal to $0$ as $i$ approaches infinity, and that c is equal to the interval $[0, frac12]$ as $i$ approaches infinity.

However i have no idea how to do this.

If someone wouldn’t mind explaining how I would do this, i would be immensely grateful.

Thanks
Tim