Explaining the error in a two-plus-two-equals-five proof

[Goopy17]

http://www.trollscientist.com/226-two-plus-two-equals-5-troll-physics/
Please go to that site and explain why they are wrong!
Thanks!

 

[micromass]

Remember that

$$x^2=y^2$$

doesn't mean that x=y. (indeed, we can have x=-y as well).

Here

$$(4-9/2)^2=(5-9/2)^2$$

doesn't mean that we can leave off the squares.

 

[mathman]

To simplify (-1/2)² = (1/2)²