October 7th, 2014, 01:12 PM  #1 
Newbie Joined: Feb 2014 Posts: 29 Thanks: 1  A parallel projection problem
Parallel projection of the rectangle ABCD at the plane is the A1B1C1D1 polygon. Find CC1 if АА1=3 сm, ВВ1 = 4 сm, DD1= 6 сm.

October 7th, 2014, 04:17 PM  #2 
Global Moderator Joined: Dec 2006 Posts: 20,835 Thanks: 2162 
Use AA1 + CC1 = BB1 + DD1.

October 8th, 2014, 08:55 AM  #3 
Newbie Joined: Feb 2014 Posts: 29 Thanks: 1 
Thanks! Makes sense to me. But how do I explain it? Is there a theorem or something?

October 10th, 2014, 08:13 AM  #4 
Global Moderator Joined: Dec 2006 Posts: 20,835 Thanks: 2162 
As ABCD is a rectangle, AC and BD have a common midpoint. Consider what happens to AC and BD separately under the parallel projection. Apply the Midpoint Connector Theorem for Trapezoids. 
October 11th, 2014, 07:08 AM  #5 
Newbie Joined: Feb 2014 Posts: 29 Thanks: 1 
Thank you! I got it now 

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