Take a simple example: .

To expand that product we take either x or y from each of the six factors.

In order to get we must take an x from each factor. That can be done in only one way.

But to get a term containing , we choose four x’s and two y’s.

We could do it as xxyxxy or yxxyxx for example. That can be done is ways.

In general, .

Just think about the number of ways to choose the terms from the n factors.

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