Trigonometry Examples

Find the Maximum/Minimum Value f(x)=1/2cos(2x)-1
Step 1
Find the first derivative of the function.
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Step 1.1
By the Sum Rule, the derivative of with respect to is .
Step 1.2
Evaluate .
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Step 1.2.1
Since is constant with respect to , the derivative of with respect to is .
Step 1.2.2
Differentiate using the chain rule, which states that is where and .
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Step 1.2.2.1
To apply the Chain Rule, set as .
Step 1.2.2.2
The derivative of with respect to is .
Step 1.2.2.3
Replace all occurrences of with .
Step 1.2.3
Since is constant with respect to , the derivative of with respect to is .
Step 1.2.4
Differentiate using the Power Rule which states that is where .
Step 1.2.5
Multiply by .
Step 1.2.6
Multiply by .
Step 1.2.7
Combine and .
Step 1.2.8
Combine and .
Step 1.2.9
Cancel the common factor of and .
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Step 1.2.9.1
Factor out of .
Step 1.2.9.2
Cancel the common factors.
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Step 1.2.9.2.1
Factor out of .
Step 1.2.9.2.2
Cancel the common factor.
Step 1.2.9.2.3
Rewrite the expression.
Step 1.2.9.2.4
Divide by .
Step 1.3
Differentiate using the Constant Rule.
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Step 1.3.1
Since is constant with respect to , the derivative of with respect to is .
Step 1.3.2
Add and .
Step 2
Find the second derivative of the function.
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Step 2.1
Since is constant with respect to , the derivative of with respect to is .
Step 2.2
Differentiate using the chain rule, which states that is where and .
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Step 2.2.1
To apply the Chain Rule, set as .
Step 2.2.2
The derivative of with respect to is .
Step 2.2.3
Replace all occurrences of with .
Step 2.3
Differentiate.
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Step 2.3.1
Since is constant with respect to , the derivative of with respect to is .
Step 2.3.2
Multiply by .
Step 2.3.3
Differentiate using the Power Rule which states that is where .
Step 2.3.4
Multiply by .
Step 3
To find the local maximum and minimum values of the function, set the derivative equal to and solve.
Step 4
Divide each term in by and simplify.
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Step 4.1
Divide each term in by .
Step 4.2
Simplify the left side.
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Step 4.2.1
Dividing two negative values results in a positive value.
Step 4.2.2
Divide by .
Step 4.3
Simplify the right side.
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Step 4.3.1
Divide by .
Step 5
Take the inverse sine of both sides of the equation to extract from inside the sine.
Step 6
Simplify the right side.
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Step 6.1
The exact value of is .
Step 7
Divide each term in by and simplify.
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Step 7.1
Divide each term in by .
Step 7.2
Simplify the left side.
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Step 7.2.1
Cancel the common factor of .
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Step 7.2.1.1
Cancel the common factor.
Step 7.2.1.2
Divide by .
Step 7.3
Simplify the right side.
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Step 7.3.1
Divide by .
Step 8
The sine function is positive in the first and second quadrants. To find the second solution, subtract the reference angle from to find the solution in the second quadrant.
Step 9
Solve for .
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Step 9.1
Simplify.
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Step 9.1.1
Multiply by .
Step 9.1.2
Add and .
Step 9.2
Divide each term in by and simplify.
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Step 9.2.1
Divide each term in by .
Step 9.2.2
Simplify the left side.
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Step 9.2.2.1
Cancel the common factor of .
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Step 9.2.2.1.1
Cancel the common factor.
Step 9.2.2.1.2
Divide by .
Step 10
The solution to the equation .
Step 11
Evaluate the second derivative at . If the second derivative is positive, then this is a local minimum. If it is negative, then this is a local maximum.
Step 12
Evaluate the second derivative.
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Step 12.1
Multiply by .
Step 12.2
The exact value of is .
Step 12.3
Multiply by .
Step 13
is a local maximum because the value of the second derivative is negative. This is referred to as the second derivative test.
is a local maximum
Step 14
Find the y-value when .
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Step 14.1
Replace the variable with in the expression.
Step 14.2
Simplify the result.
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Step 14.2.1
Simplify each term.
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Step 14.2.1.1
Multiply by .
Step 14.2.1.2
The exact value of is .
Step 14.2.1.3
Multiply by .
Step 14.2.2
To write as a fraction with a common denominator, multiply by .
Step 14.2.3
Combine and .
Step 14.2.4
Combine the numerators over the common denominator.
Step 14.2.5
Simplify the numerator.
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Step 14.2.5.1
Multiply by .
Step 14.2.5.2
Subtract from .
Step 14.2.6
Move the negative in front of the fraction.
Step 14.2.7
The final answer is .
Step 15
Evaluate the second derivative at . If the second derivative is positive, then this is a local minimum. If it is negative, then this is a local maximum.
Step 16
Evaluate the second derivative.
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Step 16.1
Cancel the common factor of .
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Step 16.1.1
Cancel the common factor.
Step 16.1.2
Rewrite the expression.
Step 16.2
Apply the reference angle by finding the angle with equivalent trig values in the first quadrant. Make the expression negative because cosine is negative in the second quadrant.
Step 16.3
The exact value of is .
Step 16.4
Multiply .
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Step 16.4.1
Multiply by .
Step 16.4.2
Multiply by .
Step 17
is a local minimum because the value of the second derivative is positive. This is referred to as the second derivative test.
is a local minimum
Step 18
Find the y-value when .
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Step 18.1
Replace the variable with in the expression.
Step 18.2
Simplify the result.
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Step 18.2.1
Simplify each term.
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Step 18.2.1.1
Cancel the common factor of .
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Step 18.2.1.1.1
Cancel the common factor.
Step 18.2.1.1.2
Rewrite the expression.
Step 18.2.1.2
Apply the reference angle by finding the angle with equivalent trig values in the first quadrant. Make the expression negative because cosine is negative in the second quadrant.
Step 18.2.1.3
The exact value of is .
Step 18.2.1.4
Multiply by .
Step 18.2.1.5
Combine and .
Step 18.2.1.6
Move the negative in front of the fraction.
Step 18.2.2
To write as a fraction with a common denominator, multiply by .
Step 18.2.3
Combine and .
Step 18.2.4
Combine the numerators over the common denominator.
Step 18.2.5
Simplify the numerator.
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Step 18.2.5.1
Multiply by .
Step 18.2.5.2
Subtract from .
Step 18.2.6
Move the negative in front of the fraction.
Step 18.2.7
The final answer is .
Step 19
These are the local extrema for .
is a local maxima
is a local minima
Step 20