2.                     
                          
              
                  
              
                  
      OCW makes the materials used in the teaching of MIT's subjects available on the Web.
       
       
       
      Learn more about how to use OpenCourseWare. Click to get started.
                          
             
         
         
                      
         
          1. 
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 蝴蝶形b是啥样子的