Question

I need to find the first five terms in the arithmetic sequence in an =(2)^n-1

n=1

I don't know now to type the n's smaller.

Answer 1

One way to indicate a subscript in plain text is to use the underscore in the same way the hat indicates a superscript.

Like so:
{a_n : a_n = 2^n -1} = {0, 1, 3, 7, 15, ...}

That is:
a_0 = 0
a_1 = 1
a_2 = 3
a_3 = 7
a_4 = 15
...

Using the array notation of most programming languages --placing the subscript in square brackets-- is also acceptable.
{a[n] : a[n] = 2^n -1} = {0, 1, 3, 7, 15, ...}

a[0] = 0
a[1] = 1
a[2] = 3
a[3] = 7
a[4] = 15
...