How do you find the critical points for #f'(x)=11+30x+18x^2+2x^3#?

1 Answer
Jul 27, 2015

Answer:

The critical numbers for #11+30x+18x^2+2x^3# are:
#-5# and #-1#.

Explanation:

I'm not sure why you called the function #f'(x)#. Since we need to take the derivative to find the critical numbers, I'll refer to the function as #g(x)#.

The critical numbers for a function #g# are the numbers in the domain of #g# at which the derivative is either #0# or does not exist. Some people use "critical point" to mean the same thing, others use it to mean a point on the graph (so it has 2 coordinates).

#g(x) = 11+30x+18x^2+2x^3#

#g'(x) = 30+36x+6x^2#

This is never undefined, so we need only find the zeros:

#6x^2+36x+30 = 0#

#6(x^2+6x+5)=0#

#6(x+5)(x+1) = 0#

#x=-5# or #x=-1#

The critical numbers for the function: #11+30x+18x^2+2x^3# are #-5# and #-1#.

If you wish to find the #y# values, you can do so.

At #x=-5#, we get:

#11+30(-5)+18(-5)^2+2(-5)^3#
(I prefer to do arithmetic with smaller numbers. Using #30 = 6*5#, we can get some multiples of #25#. So I'll regroup and use the distributive property.)

#11-6(25)+18(25)-10(25) = 11+18(25)-16(25)#

# = 11+[18-16] (25) = 11+2(25)#

# = 11 + 50 = 61#

At #x=-1#, we get:

#11-30+18-2 = 29-32 = -3#

So, if you use "critical points" to mean points on the graph (rather than points in the domain), then they will be:

#(-5, 61)# and #(-1, -3)#