From Gogeometry
Using :
- Define h1,h2,h3 and h4 the heights respectively of S1, S2, S3 and S4
- Define a the side of the square ABCD
- S1=a.h1/2
- S2=a.h2/2
- S3=a.h3/2
- S4=a.h4/2
- And h1+h3=h2+h4=a
- S1+S3=a(h1+h3)/2=a^2/2
- S2+S4=a(h2+h4)/2=a^2/2
- S=[ABCD]=a^2
- Therefore S1+S3= S2+S4=S/2