Never divide by zerio is a rule in math. what are other examples like this rule?

Question

what are rules in math that do not work.. don't divide by zero .. etc etc

Answer 1

You can't take a square root of a negetive number.

(Unless you are dealing with imaginary numbers (advanced))

Answer 2

there are no rules in math which don't work... otherwise they would not be rules , but √ x² ╪ x,  2 / x < 5 /( x+1) is not the same as 2(x+1) < 5x , if P(x) is a quadratic with roots 1 & 3 then P(x) is not always (x-1)(x-3), 10 + 5 is sometimes equal to 3 {think of time in hours on your watch} [if you use a 24 hour c**k then 20 + 7 = 3 ] , & in theory 1 / 4 is not the same as 5 / 20 { 1/4 is the representative of the equivalence class of numbers of the form [ n / 4n, n = integer] }