Calculus Examples

Find the Inflection Points y=x^3-6x^2-36x
Step 1
Write as a function.
Step 2
Find the second derivative.
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Step 2.1
Find the first derivative.
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Step 2.1.1
Differentiate.
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Step 2.1.1.1
By the Sum Rule, the derivative of with respect to is .
Step 2.1.1.2
Differentiate using the Power Rule which states that is where .
Step 2.1.2
Evaluate .
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Step 2.1.2.1
Since is constant with respect to , the derivative of with respect to is .
Step 2.1.2.2
Differentiate using the Power Rule which states that is where .
Step 2.1.2.3
Multiply by .
Step 2.1.3
Evaluate .
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Step 2.1.3.1
Since is constant with respect to , the derivative of with respect to is .
Step 2.1.3.2
Differentiate using the Power Rule which states that is where .
Step 2.1.3.3
Multiply by .
Step 2.2
Find the second derivative.
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Step 2.2.1
By the Sum Rule, the derivative of with respect to is .
Step 2.2.2
Evaluate .
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Step 2.2.2.1
Since is constant with respect to , the derivative of with respect to is .
Step 2.2.2.2
Differentiate using the Power Rule which states that is where .
Step 2.2.2.3
Multiply by .
Step 2.2.3
Evaluate .
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Step 2.2.3.1
Since is constant with respect to , the derivative of with respect to is .
Step 2.2.3.2
Differentiate using the Power Rule which states that is where .
Step 2.2.3.3
Multiply by .
Step 2.2.4
Differentiate using the Constant Rule.
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Step 2.2.4.1
Since is constant with respect to , the derivative of with respect to is .
Step 2.2.4.2
Add and .
Step 2.3
The second derivative of with respect to is .
Step 3
Set the second derivative equal to then solve the equation .
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Step 3.1
Set the second derivative equal to .
Step 3.2
Add to both sides of the equation.
Step 3.3
Divide each term in by and simplify.
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Step 3.3.1
Divide each term in by .
Step 3.3.2
Simplify the left side.
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Step 3.3.2.1
Cancel the common factor of .
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Step 3.3.2.1.1
Cancel the common factor.
Step 3.3.2.1.2
Divide by .
Step 3.3.3
Simplify the right side.
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Step 3.3.3.1
Divide by .
Step 4
Find the points where the second derivative is .
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Step 4.1
Substitute in to find the value of .
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Step 4.1.1
Replace the variable with in the expression.
Step 4.1.2
Simplify the result.
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Step 4.1.2.1
Simplify each term.
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Step 4.1.2.1.1
Raise to the power of .
Step 4.1.2.1.2
Raise to the power of .
Step 4.1.2.1.3
Multiply by .
Step 4.1.2.1.4
Multiply by .
Step 4.1.2.2
Simplify by subtracting numbers.
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Step 4.1.2.2.1
Subtract from .
Step 4.1.2.2.2
Subtract from .
Step 4.1.2.3
The final answer is .
Step 4.2
The point found by substituting in is . This point can be an inflection point.
Step 5
Split into intervals around the points that could potentially be inflection points.
Step 6
Substitute a value from the interval into the second derivative to determine if it is increasing or decreasing.
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Step 6.1
Replace the variable with in the expression.
Step 6.2
Simplify the result.
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Step 6.2.1
Multiply by .
Step 6.2.2
Subtract from .
Step 6.2.3
The final answer is .
Step 6.3
At , the second derivative is . Since this is negative, the second derivative is decreasing on the interval
Decreasing on since
Decreasing on since
Step 7
Substitute a value from the interval into the second derivative to determine if it is increasing or decreasing.
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Step 7.1
Replace the variable with in the expression.
Step 7.2
Simplify the result.
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Step 7.2.1
Multiply by .
Step 7.2.2
Subtract from .
Step 7.2.3
The final answer is .
Step 7.3
At , the second derivative is . Since this is positive, the second derivative is increasing on the interval .
Increasing on since
Increasing on since
Step 8
An inflection point is a point on a curve at which the concavity changes sign from plus to minus or from minus to plus. The inflection point in this case is .
Step 9