Question 1190095
.


Notice that the geometric progression 


            1, 2, 4, 8, 16, . . . 


with the first term 1 and the common ratio 2 has very simple formula for the sum


of the first n terms      {{{S[n]}}} = {{{2^n-1}}}


            (surely, it is a particular case of the general formula . . . )



It facilitates calculations     - - -     so,  it is useful and worth to be  MEMORIZED,

in addition to the general formula.