In the following attachment file, I have trouble trying to understand a step in sum no.2. In the Step 2 category, 3rd line, I didn't understood how they simplified from the previous step.(adsbygoogle = window.adsbygoogle || []).push({});

Also, wanted to ask one basic question. What exactly is the purpose of that sum in mathematical world? I mean how can I apply this knowledge of expanding functions in practical life?

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# Expansion of functions

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