Math Is Fun Forum

LittleBear

Member

Registered: 2014-09-21

Posts: 1

Summations

Hi! I'm pretty stamped on what to do in a problem my teacher gave us.
See, the nth term is given, however, what the n is, is not.
For example:

1+2+4+8+...+65536

where a is 1
         d is  2
        last term is 65536

Now my problem is how do I get the n in

X(sub n) ?

I have the X, but I don't know the n. Help?

Bob

Administrator

Registered: 2010-06-20

Posts: 10,643

Re: Summations

hi LittleBear

Welcome to the forum.

This sequence is called geometric.  Each term is double the one before.

The formula for the nth term is

where a = 1 and r = 2.

It's not too hard to just go on doubling and counting how many times.  But, when the numbers get tricky you can also use logs:

On a calculator you'll find a log button and also a natural log button (ln).  You can use either provided you use the same button for both logs.

http://www.mathsisfun.com/algebra/expon … ithms.html

Bob

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob