Why is division by zero is undefined or not allowed?

Answer 1

Any non-zero number divided by zero is infinity (positive or negative), but your calculator may give an error. Try it with a very small number such as 0.000001, as you make it smaller the result will become larger. The reason is that for a given number, you can put 0 into it as many times as you want - an infinite amount.

However, this will give us many problems with division. For any other number, division has very useful properties. If we have some unknown number called 'x', and we have an equation that says: x/3 = 5/3, then we can deduce that x=5. Another example: x/4 = 7/4. We can deduce that x=7. But when we allow division by zero, this property is lost: 3/0 = infinity = 4/0, but 3 is not equal to 4. We also lose many other useful properties if we allow division by zero. However, we do almost allow division by zero. This is done by taking what is known as a limit as the divisor, x, tends to zero (we write x→0), and this is an integral part of calculus.

0/0 is a special case of division by zero. Notice that for any non-zero number, let's call it 'y', we get y/y = 1. But we say that 0/0 is undefined, or indeterminate. We can still take limits when something looks like it might be equal to 0/0. For example, the limit of sine(x)/x as x→0. We know that sine(x) = 0 when x=0, but it can be proven that sine(x)/x →1 as x→0. But we can also prove that x2/x → 0 as x→0. So the result is different depending of different situations.