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Intuitively, the limit of f (x) as x approaches c is the value that f (x) approaches as x approaches c. For example, the limit of f (x) = x ^{2} + 2 as x approaches 2 is 6:
As x gets closer and closer to 2, f (x) gets closer and closer to 6. In mathematical notation, we can represent this as
f (x) = 6 or x ^{2}+2 = 6 |
Note that we've only been talking about what happens to f (x) as x approaches c , and not about what happens when x equals c. The truth is that when we're looking for limits, we don't care what happens to f (x) when x actually equals c -- we're only concerned with its behavior as x gets closer and closer to c. Consider the following piecewise-defined function:
f (x) = |
Note that this function looks just like the function f (x) = x ^{2} + 2 , except that f (2) = 9 instead of 6. What happens when we try to find
f (x) ? |
We see that the limit is again 6. Once again, this is because the limit doesn't care what happens when x = c ! As long as two functions approach the same value as x approaches c , their limits will be the same.
The standard limit that we've been talking about is a two-sided limit. It is considered two-sided because we get the same value for the limit whether we let x approach c "from the left" (i.e. from values of x less than c )