00:01
In this exercise we have our function f of x equal to 2x cubed minus 3x squared minus 12x.
00:12
Let's compute the derivative of this function.
00:17
The derivative is 6x squared minus 6x minus 12.
00:24
This one can be written as 6 multiplied by x squared minus x minus 2.
00:35
Perfect.
00:36
Now we need to find the zeros.
00:40
Ok, f prime of x is equal to 0 if and only if, thanks to the quadratic formula, x is equal to 1 plus or minus square root of 9 over 2.
00:57
So we get 1 plus 3 over 2 which is 2, 1 minus 3 over 2 which is negative 1.
01:07
Perfect.
01:08
Ok, at this point what do we know? we know that this is going to be the sign chart for f prime.
01:18
Well, we know that f prime is 0 at negative 1 and 2.
01:24
Negative here and positive here.
01:28
Perfect.
01:30
So this thing shows that the local extreme of f are x equal to negative 1 and 2.
01:49
Moreover, we know that f is increasing where f prime is positive.
01:55
So, increasing on the union of the intervals negative infinity, negative 1 and 2 comma infinity.
02:06
And clearly f is decreasing on the complement.
02:12
So, on negative 1 comma 2...