how do you prove 2+2=5?

Answer 1

Let a=b.
Square both sides: a²=b²
a²-b²=0 ... [1]
(a+b)(a-b)=0 ... [2]
a²-b×b=0
a²-a×b=0 ... a=b, so substitute a instead of b.
a(a-b)=0 ... [3]
[2] and [3] are the same expressions: a(a-b)=(a+b)(a-b)
divide both sides by (a-b): a=a+b
a equals b: a=a+a
divide both sides by a, 1=1+1
1=2
Therefore, 1+1=1+2=3

Answer 2

You can't. If you could the entire banking system, all the computers in the world would collapse.

If you do either one of the following assumptions then you can prove it:

Let 5 = 4
or Let 2 = 2.5

If you make these assumptions then you're just fine.