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This image was selected as picture of the month on the Mathematics Portal for October 2011 

This image appeared on Wikipedia's Main Page in the Did you know? column on 26 April 2013. 

Summary
Description 
English: Animation of a proof of Pythagoras' theorem, showing how by rearranging triangles the areas a^{2} + b^{2} and c^{2} can be shown to be the same. The area of the outer square never changes, and the total area of the four right triangles is the same at both the beginning and the end, therefore the black area at the beginning, a^{2} + b^{2}, must equal the black area at the end, c^{2}. The angle of the triangles is arbitrary, therefore this works as a general proof. To be more explicit...
The situation at the start is:
Area_of_the_Outer_Square  (4 x Area_of_a_Triangle) = a^{2} + b^{2}
...and the situation at the end is:
Area_of_the_Outer_Square  (4 x Area_of_a_Triangle) = c^{2}
The values on both lefthand sides of these two equations are exactly the same (only positions have changed) therefore the values on the righthand sides must also be exactly the same: a ^{2} + b ^{2} = c ^{2}.

Date 

Source 
Own work 
Author 
JohnBlackburne 
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