This reminds me of this video about why we need to be very careful when inspecting visual proofs: https://www.youtube.com/watch?v=VYQVlVoWoPY (it includes a "proof" that pi equals exactly 4).
In this case, as someone else pointed out below, this proof has unjustified assumptions in it (at least it assumes that b < a).
In this case, as someone else pointed out below, this proof has unjustified assumptions in it (at least it assumes that b < a).