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I was trying to figure out the step between these two equal expression:

$(2k)\cdot 2^{k+1} = k\cdot 2^{k+2}$

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    @BrianM.Scott: that's it!! I was thinking how the k goes out. It is the associative law!! Thank you all.2012-04-29

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HINT A number raised to an exponent tells you how many times to multiply that number by itself.

For example: $2^{k}=2*2*...*2$ k-times, so $2*2^{k}=2*(2*2*...*2)$ a total of $k+1$ times so $2*2^{k}=2^{k+1}$