Find any critical numbers of the function. (Enter your answers as a comma-separated list.)

g(x) = x^7 − 7x^5

Honestly, I have no idea how to find the critical number!

User Icon for Reiny Reiny answered
12 years ago

by critical numbers, they mean

the turning points, (maximim and minimum points)
any points of inflection
the x and the y -intercepts

so for y-intercept, let x = 0
so g(0) = 0, the origin (0,0) is both an x and a y intercept.
for more x-intercepts , let y = 0
x^7 - 7x^5 = 0
x^5(x^2 - 7) = 0
so x = 0 , x = ± √7

g'(x) = 7x^6 - 35x^4 = 0 for max/mins
7x^4(x^2 - 5) = 0
x = 0 or x = ± √5

g''(x) = 42x^5 - 140x^3
= 0 for any points of inflection
14x^3(3x^2 - 10) = 0
x = 0 or x = ± √(10/3)

since (0,0) is both a turning point and a point of inflection, It is neither a maximim nor a minimum

so you have 4 other different x values to sub back into the original equation to find the corresponding y value of the points,
I will leave that up to you

User Icon for Reza Reza answered
12 years ago

Thank you I really appreciate your help :)

User Icon for Explain Bot Explain Bot answered
11 months ago

To find the critical numbers of a function, you need to follow these steps:

1. Calculate the derivative of the function.
2. Set the derivative equal to zero and solve for x.
3. The values of x obtained in step 2 are the critical numbers.

Let's work through the steps for the given function g(x) = x^7 − 7x^5.

Step 1: Calculate the derivative of g(x).
To find the derivative of g(x), you can use the power rule, which states that the derivative of x^n is n * x^(n-1).

Taking the derivative of g(x) = x^7 − 7x^5:
g'(x) = 7x^(7-1) - 5(7)x^(5-1)
= 7x^6 - 35x^4

Step 2: Set the derivative equal to zero and solve for x.
Setting g'(x) = 0:
7x^6 - 35x^4 = 0

To solve this equation, you can factor out the common factor x^4:
x^4(7x^2 - 35) = 0

Now, we have two possibilities:
1. x^4 = 0 => x = 0
2. 7x^2 - 35 = 0 => 7x^2 = 35 => x^2 = 5 => x = ±√5

Step 3: Determine the critical numbers.
The critical numbers are the values of x obtained in step 2, which are:
- x = 0
- x = √5
- x = -√5

So, the critical numbers of the given function g(x) = x^7 − 7x^5 are 0, √5, and -√5.

Please note that these are potential critical numbers. To confirm if they are indeed critical points, you should check if the second derivative is positive or negative at each point.