Thank you for the opportunity to help you with your question!
For these types of problems, the main thing is to notice a certain pattern. You can tell that the left-hand side goes up by one each time. To get the right hand side, you multiply the left-hand side by 'one more than the left hand side'.
So, once you get to 9, you just multiply it by 10 to get the right hand side.
Please let me know if you need any clarification. I'm always happy to answer your questions.
Sep 7th, 2015
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