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## 'Double Trouble' printed from http://nrich.maths.org/

### Charlie has been adding fractions in the sequence $\frac{1}{2}, \frac{1}{4}, \frac{1}{8}, \dots$ where each fraction is half the previous one:

$$\frac{1}{2} + \frac{1}{4} $$ $$\frac{1}{2} + \frac{1}{4} + \frac{1}{8}$$ $$\frac{1}{2} + \frac{1}{4} + \frac{1}{8} +\frac{1}{16}$$

Work out the answers to Charlie's sums. What do you notice?

Will the pattern continue?

How do you know?

Try writing an expression for $$\frac{1}{2} + \frac{1}{4} + \frac{1}{8} + \dots + \frac{1}{2^n}$$

Could you convince someone else that your expression is correct for all values of $n$?

Charlie drew a diagram to try to explain what was going on:

Use Charlie's diagram to explain why $$\frac{1}{2} + \frac{1}{4} + \frac{1}{8} + \dots + \frac{1}{2^n} = 1-\frac{1}{2^n} = \frac{2^n-1}{2^n}$$

### Alison has been adding numbers in the sequence $1, 2, 4, 8, \dots$ where each number is twice the previous one:

$$1 + 2$$ $$1 + 2 + 4$$ $$1 + 2 + 4 + 8$$

Work out the answers to Alison's sums. What do you notice?

Will the pattern continue?

How do you know?

Try writing an expression for $$1 + 2 + 4 + \dots + 2^n$$

Could you convince someone else that your expression is correct for all values of $n$?

Alison drew a diagram to try to explain what was going on:

Can you use Alison's diagram to explain why $$1 + 2 + 4 + \dots + 2^n = 2^{n+1}-1$$