When you install the software you will get several files and folders. For example if you have windows 7, you will get Autodesk_Autocad_2010_win32_x86_dyn.zip and Autodesk_Autocad_2010_win32_x64_dyn.zip If you have vista, you will get Autodesk_Autocad_2010_win32_x86_dyn.zip and Autodesk_Autocad_2010_win32_x64_dyn.zip If you have windows 8, you will get Autodesk_Autocad_2010_win32_x86_dyn.zip and Autodesk_Autocad_2010_win32_x64_dyn.zip Extract the files that you want to use. In my case, I extract them into a folder. For example: C:\Program Files\Autodesk\Autocad\2010 1. Create a folder called “sfc” 2. Copy and Paste the files inside the “sfc” folder. 3. Run the following command in the Command Prompt. sfc /scannow And you will get a screen that says, Successfully completed repair. You don’t need to do anything now. For information: Q: Simple algebraic complexity proof: $\binom{n}{2} = \binom{n-1}{2} + n$ I’m having trouble seeing how this is true. The definition of a binomial coefficient is: $\binom{n}{k} = \frac{n!}{k!(n-k)!}$ And if you look at the left hand side, $\binom{n}{k}$ is an