let f(x) = sin(x^3) Find the critical numbers of f on the interval [-2,2]

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nick-teal's profile pic

nick-teal | High School Teacher | (Level 3) Adjunct Educator

Posted on

There are actually a few more critical numbers.  We want to find all ways that

`cos(x^3) * 3x^2 = 0`

This is true when x = 0.

We can graph this equation to see all the spots where it is approximately zero.

There are 7 spots where our function equals zero.

So we need to find all ways that `cos(x^3) = 0`

Cos equals zero when at +-pi/2 and +-3pi/2 and +-5pi/2, so we can set x^3 equal to those and find more x values where f'(x) = 0.

`x^3 = pi/2`

`x = (pi/2)^(1/3) ~~ 1.163`

`x^3 = -pi/2`

`x = (-pi/2)^(1/3) ~~ -1.163`

---

`x^3 = (3pi)/2`

`x = ((3pi)/2)^(1/3) ~~ 1.676`

`x^3 = -(3pi)/2`

`x = ((-3pi)/2)^(1/3) ~~ -1.676`

---

`x^3 = ((5pi)/2)`

`x = ((5pi)/2)^(1/3) ~~ 1.98`

`x ~~ -1.98`

So those 6 answers are the other spots where f'(x) = 0.

Our set of solutions is x = +- 1.98, +- 1.676, +- 1.163, and 0.

Hope this helps!

gsenviro's profile pic

gsenviro | College Teacher | (Level 1) Educator Emeritus

Posted on

A number 'a' is a critical number of f in a given interval if f'(a)=0 or f' is undefined at x=a.

Here, f(x) = sin (x^3)

Differentiating it with respect to x,

f'(x) = 3(x^2)* cos(x^3)

In the interval [-2,2], f' = 0 at x=0.

At all other values of x, cosine will be non-zero and will be defined, so will be the term 3 x^2.

Thus, 0 is the critical number of f in the given interval [-2,2].

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