Question

Find S11 for 1 + 2 + 4 + 8 +...

Answer 1

notice that this is an arithmatic sequence.
each number increases by "2"
so, the common difference = d = 2
first number = \(a_0\) =1

we use the formula for n^th number:
\[S_n={n\over2}[a_0+(n-1)d]\qquad n=11\]