Portal:High school mathematics/Mathematical errors

Classic Maths Mistakes
Here I am collecting mistakes in maths which do not work. Ok, let's start the (so far, small) list!

MISTAKE 1: (b+1)²=b²+1

MISTAKE 2: (a+b)^2=a^2+b^2

How to add new mistakes
Just put in your maths mistake and put 'MISTAKE X' above it. X is the number of the mistake.