$ABCD$ABCD is a parallelogram with $AE=FC$AE=FC. Prove that $DE$DE=$FB$FB.
In $\triangle AED$△AED and $\triangle CFB$△CFB we have :
$ABCD$ABCD is a parallelogram, with $AY$AY=$XC$XC.
Prove that the two triangles $\triangle AYB$△AYB and $\triangle CXD$△CXD are congruent.
In the diagram, $ABCD$ABCD is a square, and $AE=CF$AE=CF. Prove that $AF=EC$AF=EC.