In algebra, a very common mistake you can see learner making is the below:
52 - 22 = (5 - 2)2 = 32 = 9
This is very interesting.
It seems to be correct. That is the problem with this form of mathematical operation.
If you are aware that A2 - B2 = (A + B)(A - B), then this mistake will not occur.
It is this slip-of-the-mind type of human error.
It occurs when you are not alert or too tired after too many assignment quesions.
The correct answer is 52 - 22 = 25 - 4 = 21.
Or (5 + 2)(5 - 2) = 7 x 3 = 21.
This is why maths is interesting. It catches you when you are not alert!