## Finding extreme values

So I know how to find extreme value when it is a closed interval
for example

f(x) = x^2 - 1, -1 <= x <= 2
in this i would first find the critical point. and then i would compare f(critical point) and f(-1) and f(2) and then find the maximum and minimum values that way.

but my question is how to find extreme values when it is not a closed interval. Right now what I am doing is I plot the graph on my calculator and look at the max and the minimum values. But i am sure thats not the correct way of doing it. I'm sure there is some way to find the max and the min algebraically.

here is an example of a problem without closed intervals
f(x) = 2x^2 - 8x + 9

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 anyone.......???
 Have you tried the second derivative test?

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