MathCAD Spreadsheets - Need Help

I have attached three files.  FFT1, FFT4, and FFT5.  I'm trying to learn how to do a fast fourier transform.  I begin with a 1 microsecond sampling rate, which provide a max. nyquist frequency of 500 kHz.  I multiply the number of points in FFT4 by a factor of 10, so the sampling rate becomes 0.1 microsecond, which provides a max Nyquist frequency of 5 MegaHz.  Then I again multiply the sampling frequency by 10 and produce a sampling rate of 0.01 microseconds and a Nyquist frequency of 50.  Can anyone take a look and confirm this theory?  

How do I reproduce a segment of the output FFT?  I want to reproduce the segment above n=50 MHz in the final spreadsheet.  

How do I reproduce the segment beginning with n=0 and ending with n=50 in the FFT5 spreadsheet.

I want to reproduce the original waveform as well..

Any assistance understanding this software would be welcome.

I also need to know what dft is doing relative to FFT.  I know it uses fewer points, are there any  deficiencies?

I need to know that what I'm doing is correct.