Calculus Examples

$x^{3} - 14 x^{2} + 49 x$

Step 1

Write $x^{3} - 14 x^{2} + 49 x$ as a function.

$f (x) = x^{3} - 14 x^{2} + 49 x$

Step 2

Find the first derivative of the function.

Tap for more steps...

Step 2.1

Differentiate.

Tap for more steps...

Step 2.1.1

By the Sum Rule, the derivative of $x^{3} - 14 x^{2} + 49 x$ with respect to $x$ is $\frac{d}{d x} [x^{3}] + \frac{d}{d x} [- 14 x^{2}] + \frac{d}{d x} [49 x]$ .

$\frac{d}{d x} [x^{3}] + \frac{d}{d x} [- 14 x^{2}] + \frac{d}{d x} [49 x]$

Step 2.1.2

Differentiate using the Power Rule which states that $\frac{d}{d x} [x^{n}]$ is $n x^{n - 1}$ where $n = 3$ .

$3 x^{2} + \frac{d}{d x} [- 14 x^{2}] + \frac{d}{d x} [49 x]$

Step 2.2

Evaluate $\frac{d}{d x} [- 14 x^{2}]$ .

Tap for more steps...

Step 2.2.1

Since $- 14$ is constant with respect to $x$ , the derivative of $- 14 x^{2}$ with respect to $x$ is $- 14 \frac{d}{d x} [x^{2}]$ .

$3 x^{2} - 14 \frac{d}{d x} [x^{2}] + \frac{d}{d x} [49 x]$

Step 2.2.2

Differentiate using the Power Rule which states that $\frac{d}{d x} [x^{n}]$ is $n x^{n - 1}$ where $n = 2$ .

$3 x^{2} - 14 (2 x) + \frac{d}{d x} [49 x]$

Step 2.2.3

Multiply $2$ by $- 14$ .

$3 x^{2} - 28 x + \frac{d}{d x} [49 x]$

Step 2.3

Evaluate $\frac{d}{d x} [49 x]$ .

Tap for more steps...

Step 2.3.1

Since $49$ is constant with respect to $x$ , the derivative of $49 x$ with respect to $x$ is $49 \frac{d}{d x} [x]$ .

$3 x^{2} - 28 x + 49 \frac{d}{d x} [x]$

Step 2.3.2

Differentiate using the Power Rule which states that $\frac{d}{d x} [x^{n}]$ is $n x^{n - 1}$ where $n = 1$ .

$3 x^{2} - 28 x + 49 \cdot 1$

Step 2.3.3

Multiply $49$ by $1$ .

$3 x^{2} - 28 x + 49$

Step 3

Find the second derivative of the function.

Tap for more steps...

Step 3.1

By the Sum Rule, the derivative of $3 x^{2} - 28 x + 49$ with respect to $x$ is $\frac{d}{d x} [3 x^{2}] + \frac{d}{d x} [- 28 x] + \frac{d}{d x} [49]$ .

$f'' (x) = \frac{d}{d x} (3 x^{2}) + \frac{d}{d x} (- 28 x) + \frac{d}{d x} (49)$

Step 3.2

Evaluate $\frac{d}{d x} [3 x^{2}]$ .

Tap for more steps...

Step 3.2.1

Since $3$ is constant with respect to $x$ , the derivative of $3 x^{2}$ with respect to $x$ is $3 \frac{d}{d x} [x^{2}]$ .

$f'' (x) = 3 \frac{d}{d x} (x^{2}) + \frac{d}{d x} (- 28 x) + \frac{d}{d x} (49)$

Step 3.2.2

Differentiate using the Power Rule which states that $\frac{d}{d x} [x^{n}]$ is $n x^{n - 1}$ where $n = 2$ .

$f'' (x) = 3 (2 x) + \frac{d}{d x} (- 28 x) + \frac{d}{d x} (49)$

Step 3.2.3

Multiply $2$ by $3$ .

$f'' (x) = 6 x + \frac{d}{d x} (- 28 x) + \frac{d}{d x} (49)$

Step 3.3

Evaluate $\frac{d}{d x} [- 28 x]$ .

Tap for more steps...

Step 3.3.1

Since $- 28$ is constant with respect to $x$ , the derivative of $- 28 x$ with respect to $x$ is $- 28 \frac{d}{d x} [x]$ .

$f'' (x) = 6 x - 28 \frac{d}{d x} x + \frac{d}{d x} (49)$

Step 3.3.2

Differentiate using the Power Rule which states that $\frac{d}{d x} [x^{n}]$ is $n x^{n - 1}$ where $n = 1$ .

$f'' (x) = 6 x - 28 \cdot 1 + \frac{d}{d x} (49)$

Step 3.3.3

Multiply $- 28$ by $1$ .

$f'' (x) = 6 x - 28 + \frac{d}{d x} (49)$

Step 3.4

Differentiate using the Constant Rule.

Tap for more steps...

Step 3.4.1

Since $49$ is constant with respect to $x$ , the derivative of $49$ with respect to $x$ is $0$ .

$f'' (x) = 6 x - 28 + 0$

Step 3.4.2

Add $6 x - 28$ and $0$ .

$f'' (x) = 6 x - 28$

Step 4

To find the local maximum and minimum values of the function, set the derivative equal to $0$ and solve.

$3 x^{2} - 28 x + 49 = 0$

Step 5

Find the first derivative.

Tap for more steps...

Step 5.1

Find the first derivative.

Tap for more steps...

Step 5.1.1

Differentiate.

Tap for more steps...

Step 5.1.1.1

By the Sum Rule, the derivative of $x^{3} - 14 x^{2} + 49 x$ with respect to $x$ is $\frac{d}{d x} [x^{3}] + \frac{d}{d x} [- 14 x^{2}] + \frac{d}{d x} [49 x]$ .

$\frac{d}{d x} [x^{3}] + \frac{d}{d x} [- 14 x^{2}] + \frac{d}{d x} [49 x]$

Step 5.1.1.2

Differentiate using the Power Rule which states that $\frac{d}{d x} [x^{n}]$ is $n x^{n - 1}$ where $n = 3$ .

$3 x^{2} + \frac{d}{d x} [- 14 x^{2}] + \frac{d}{d x} [49 x]$

Step 5.1.2

Evaluate $\frac{d}{d x} [- 14 x^{2}]$ .

Tap for more steps...

Step 5.1.2.1

Since $- 14$ is constant with respect to $x$ , the derivative of $- 14 x^{2}$ with respect to $x$ is $- 14 \frac{d}{d x} [x^{2}]$ .

$3 x^{2} - 14 \frac{d}{d x} [x^{2}] + \frac{d}{d x} [49 x]$

Step 5.1.2.2

Differentiate using the Power Rule which states that $\frac{d}{d x} [x^{n}]$ is $n x^{n - 1}$ where $n = 2$ .

$3 x^{2} - 14 (2 x) + \frac{d}{d x} [49 x]$

Step 5.1.2.3

Multiply $2$ by $- 14$ .

$3 x^{2} - 28 x + \frac{d}{d x} [49 x]$

Step 5.1.3

Evaluate $\frac{d}{d x} [49 x]$ .

Tap for more steps...

Step 5.1.3.1

Since $49$ is constant with respect to $x$ , the derivative of $49 x$ with respect to $x$ is $49 \frac{d}{d x} [x]$ .

$3 x^{2} - 28 x + 49 \frac{d}{d x} [x]$

Step 5.1.3.2

Differentiate using the Power Rule which states that $\frac{d}{d x} [x^{n}]$ is $n x^{n - 1}$ where $n = 1$ .

$3 x^{2} - 28 x + 49 \cdot 1$

Step 5.1.3.3

Multiply $49$ by $1$ .

$f' (x) = 3 x^{2} - 28 x + 49$

Step 5.2

The first derivative of $f (x)$ with respect to $x$ is $3 x^{2} - 28 x + 49$ .

$3 x^{2} - 28 x + 49$

Step 6

Set the first derivative equal to $0$ then solve the equation $3 x^{2} - 28 x + 49 = 0$ .

Tap for more steps...

Step 6.1

Set the first derivative equal to $0$ .

$3 x^{2} - 28 x + 49 = 0$

Step 6.2

Factor by grouping.

Tap for more steps...

Step 6.2.1

For a polynomial of the form $a x^{2} + b x + c$ , rewrite the middle term as a sum of two terms whose product is $a \cdot c = 3 \cdot 49 = 147$ and whose sum is $b = - 28$ .

Tap for more steps...

Step 6.2.1.1

Factor $- 28$ out of $- 28 x$ .

$3 x^{2} - 28 x + 49 = 0$

Step 6.2.1.2

Rewrite $- 28$ as $- 7$ plus $- 21$

$3 x^{2} + (- 7 - 21) x + 49 = 0$

Step 6.2.1.3

Apply the distributive property.

$3 x^{2} - 7 x - 21 x + 49 = 0$

Step 6.2.2

Factor out the greatest common factor from each group.

Tap for more steps...

Step 6.2.2.1

Group the first two terms and the last two terms.

$(3 x^{2} - 7 x) - 21 x + 49 = 0$

Step 6.2.2.2

Factor out the greatest common factor (GCF) from each group.

$x (3 x - 7) - 7 (3 x - 7) = 0$

Step 6.2.3

Factor the polynomial by factoring out the greatest common factor, $3 x - 7$ .

$(3 x - 7) (x - 7) = 0$

Step 6.3

If any individual factor on the left side of the equation is equal to $0$ , the entire expression will be equal to $0$ .

$3 x - 7 = 0$

$x - 7 = 0$

Step 6.4

Set $3 x - 7$ equal to $0$ and solve for $x$ .

Tap for more steps...

Step 6.4.1

Set $3 x - 7$ equal to $0$ .

$3 x - 7 = 0$

Step 6.4.2

Solve $3 x - 7 = 0$ for $x$ .

Tap for more steps...

Step 6.4.2.1

Add $7$ to both sides of the equation.

$3 x = 7$

Step 6.4.2.2

Divide each term in $3 x = 7$ by $3$ and simplify.

Tap for more steps...

Step 6.4.2.2.1

Divide each term in $3 x = 7$ by $3$ .

$\frac{3 x}{3} = \frac{7}{3}$

Step 6.4.2.2.2

Simplify the left side.

Tap for more steps...

Step 6.4.2.2.2.1

Cancel the common factor of $3$ .

Tap for more steps...

Step 6.4.2.2.2.1.1

Cancel the common factor.

$\frac{3 x}{3} = \frac{7}{3}$

Step 6.4.2.2.2.1.2

Divide $x$ by $1$ .

$x = \frac{7}{3}$

Step 6.5

Set $x - 7$ equal to $0$ and solve for $x$ .

Tap for more steps...

Step 6.5.1

Set $x - 7$ equal to $0$ .

$x - 7 = 0$

Step 6.5.2

Add $7$ to both sides of the equation.

$x = 7$

Step 6.6

The final solution is all the values that make $(3 x - 7) (x - 7) = 0$ true.

$x = \frac{7}{3}, 7$

Step 7

Find the values where the derivative is undefined.

Tap for more steps...

Step 7.1

The domain of the expression is all real numbers except where the expression is undefined. In this case, there is no real number that makes the expression undefined.

Step 8

Critical points to evaluate.

$x = \frac{7}{3}, 7$

Step 9

Evaluate the second derivative at $x = \frac{7}{3}$ . If the second derivative is positive, then this is a local minimum. If it is negative, then this is a local maximum.

$6 (\frac{7}{3}) - 28$

Step 10

Evaluate the second derivative.

Tap for more steps...

Step 10.1

Simplify each term.

Tap for more steps...

Step 10.1.1

Cancel the common factor of $3$ .

Tap for more steps...

Step 10.1.1.1

Factor $3$ out of $6$ .

$3 (2) \frac{7}{3} - 28$

Step 10.1.1.2

Cancel the common factor.

$3 \cdot 2 \frac{7}{3} - 28$

Step 10.1.1.3

Rewrite the expression.

$2 \cdot 7 - 28$

Step 10.1.2

Multiply $2$ by $7$ .

$14 - 28$

Step 10.2

Subtract $28$ from $14$ .

$- 14$

Step 11

$x = \frac{7}{3}$ is a local maximum because the value of the second derivative is negative. This is referred to as the second derivative test.

$x = \frac{7}{3}$ is a local maximum

Step 12

Find the y-value when $x = \frac{7}{3}$ .

Tap for more steps...

Step 12.1

Replace the variable $x$ with $\frac{7}{3}$ in the expression.

$f (\frac{7}{3}) = {(\frac{7}{3})}^{3} - 14 {(\frac{7}{3})}^{2} + 49 (\frac{7}{3})$

Step 12.2

Simplify the result.

Tap for more steps...

Step 12.2.1

Simplify each term.

Tap for more steps...

Step 12.2.1.1

Apply the product rule to $\frac{7}{3}$ .

$f (\frac{7}{3}) = \frac{7^{3}}{3^{3}} - 14 {(\frac{7}{3})}^{2} + 49 (\frac{7}{3})$

Step 12.2.1.2

Raise $7$ to the power of $3$ .

$f (\frac{7}{3}) = \frac{343}{3^{3}} - 14 {(\frac{7}{3})}^{2} + 49 (\frac{7}{3})$

Step 12.2.1.3

Raise $3$ to the power of $3$ .

$f (\frac{7}{3}) = \frac{343}{27} - 14 {(\frac{7}{3})}^{2} + 49 (\frac{7}{3})$

Step 12.2.1.4

Apply the product rule to $\frac{7}{3}$ .

$f (\frac{7}{3}) = \frac{343}{27} - 14 \frac{7^{2}}{3^{2}} + 49 (\frac{7}{3})$

Step 12.2.1.5

Raise $7$ to the power of $2$ .

$f (\frac{7}{3}) = \frac{343}{27} - 14 \frac{49}{3^{2}} + 49 (\frac{7}{3})$

Step 12.2.1.6

Raise $3$ to the power of $2$ .

$f (\frac{7}{3}) = \frac{343}{27} - 14 (\frac{49}{9}) + 49 (\frac{7}{3})$

Step 12.2.1.7

Multiply $- 14 (\frac{49}{9})$ .

Tap for more steps...

Step 12.2.1.7.1

Combine $- 14$ and $\frac{49}{9}$ .

$f (\frac{7}{3}) = \frac{343}{27} + \frac{- 14 \cdot 49}{9} + 49 (\frac{7}{3})$

Step 12.2.1.7.2

Multiply $- 14$ by $49$ .

$f (\frac{7}{3}) = \frac{343}{27} + \frac{- 686}{9} + 49 (\frac{7}{3})$

Step 12.2.1.8

Move the negative in front of the fraction.

$f (\frac{7}{3}) = \frac{343}{27} - \frac{686}{9} + 49 (\frac{7}{3})$

Step 12.2.1.9

Multiply $49 (\frac{7}{3})$ .

Tap for more steps...

Step 12.2.1.9.1

Combine $49$ and $\frac{7}{3}$ .

$f (\frac{7}{3}) = \frac{343}{27} - \frac{686}{9} + \frac{49 \cdot 7}{3}$

Step 12.2.1.9.2

Multiply $49$ by $7$ .

$f (\frac{7}{3}) = \frac{343}{27} - \frac{686}{9} + \frac{343}{3}$

Step 12.2.2

Find the common denominator.

Tap for more steps...

Step 12.2.2.1

Multiply $\frac{686}{9}$ by $\frac{3}{3}$ .

$f (\frac{7}{3}) = \frac{343}{27} - (\frac{686}{9} \cdot \frac{3}{3}) + \frac{343}{3}$

Step 12.2.2.2

Multiply $\frac{686}{9}$ by $\frac{3}{3}$ .

$f (\frac{7}{3}) = \frac{343}{27} - \frac{686 \cdot 3}{9 \cdot 3} + \frac{343}{3}$

Step 12.2.2.3

Multiply $\frac{343}{3}$ by $\frac{9}{9}$ .

$f (\frac{7}{3}) = \frac{343}{27} - \frac{686 \cdot 3}{9 \cdot 3} + \frac{343}{3} \cdot \frac{9}{9}$

Step 12.2.2.4

Multiply $\frac{343}{3}$ by $\frac{9}{9}$ .

$f (\frac{7}{3}) = \frac{343}{27} - \frac{686 \cdot 3}{9 \cdot 3} + \frac{343 \cdot 9}{3 \cdot 9}$

Step 12.2.2.5

Reorder the factors of $9 \cdot 3$ .

$f (\frac{7}{3}) = \frac{343}{27} - \frac{686 \cdot 3}{3 \cdot 9} + \frac{343 \cdot 9}{3 \cdot 9}$

Step 12.2.2.6

Multiply $3$ by $9$ .

$f (\frac{7}{3}) = \frac{343}{27} - \frac{686 \cdot 3}{27} + \frac{343 \cdot 9}{3 \cdot 9}$

Step 12.2.2.7

Multiply $3$ by $9$ .

$f (\frac{7}{3}) = \frac{343}{27} - \frac{686 \cdot 3}{27} + \frac{343 \cdot 9}{27}$

Step 12.2.3

Combine the numerators over the common denominator.

$f (\frac{7}{3}) = \frac{343 - 686 \cdot 3 + 343 \cdot 9}{27}$

Step 12.2.4

Simplify each term.

Tap for more steps...

Step 12.2.4.1

Multiply $- 686$ by $3$ .

$f (\frac{7}{3}) = \frac{343 - 2058 + 343 \cdot 9}{27}$

Step 12.2.4.2

Multiply $343$ by $9$ .

$f (\frac{7}{3}) = \frac{343 - 2058 + 3087}{27}$

Step 12.2.5

Simplify by adding and subtracting.

Tap for more steps...

Step 12.2.5.1

Subtract $2058$ from $343$ .

$f (\frac{7}{3}) = \frac{- 1715 + 3087}{27}$

Step 12.2.5.2

Add $- 1715$ and $3087$ .

$f (\frac{7}{3}) = \frac{1372}{27}$

Step 12.2.6

The final answer is $\frac{1372}{27}$ .

$y = \frac{1372}{27}$

Step 13

Evaluate the second derivative at $x = 7$ . If the second derivative is positive, then this is a local minimum. If it is negative, then this is a local maximum.

$6 (7) - 28$

Step 14

Evaluate the second derivative.

Tap for more steps...

Step 14.1

Multiply $6$ by $7$ .

$42 - 28$

Step 14.2

Subtract $28$ from $42$ .

$14$

Step 15

$x = 7$ is a local minimum because the value of the second derivative is positive. This is referred to as the second derivative test.

$x = 7$ is a local minimum

Step 16

Find the y-value when $x = 7$ .

Tap for more steps...

Step 16.1

Replace the variable $x$ with $7$ in the expression.

$f (7) = {(7)}^{3} - 14 {(7)}^{2} + 49 (7)$

Step 16.2

Simplify the result.

Tap for more steps...

Step 16.2.1

Simplify each term.

Tap for more steps...

Step 16.2.1.1

Raise $7$ to the power of $3$ .

$f (7) = 343 - 14 {(7)}^{2} + 49 (7)$

Step 16.2.1.2

Raise $7$ to the power of $2$ .

$f (7) = 343 - 14 \cdot 49 + 49 (7)$

Step 16.2.1.3

Multiply $- 14$ by $49$ .

$f (7) = 343 - 686 + 49 (7)$

Step 16.2.1.4

Multiply $49$ by $7$ .

$f (7) = 343 - 686 + 343$

Step 16.2.2

Simplify by adding and subtracting.

Tap for more steps...

Step 16.2.2.1

Subtract $686$ from $343$ .

$f (7) = - 343 + 343$

Step 16.2.2.2

Add $- 343$ and $343$ .

$f (7) = 0$

Step 16.2.3

The final answer is $0$ .

$y = 0$

Step 17

These are the local extrema for $f (x) = x^{3} - 14 x^{2} + 49 x$ .

$(\frac{7}{3}, \frac{1372}{27})$ is a local maxima

$(7, 0)$ is a local minima

Step 18