Find the Maximum/Minimum Value f(x)=9x^3+6x^2-7

$f (x) = 9 x^{3} + 6 x^{2} - 7$

Step 1

Find the first derivative of the function.

By the Sum Rule, the derivative of $9 x^{3} + 6 x^{2} - 7$ with respect to $x$ is $\frac{d}{d x} [9 x^{3}] + \frac{d}{d x} [6 x^{2}] + \frac{d}{d x} [- 7]$ .

$\frac{d}{d x} [9 x^{3}] + \frac{d}{d x} [6 x^{2}] + \frac{d}{d x} [- 7]$

Evaluate $\frac{d}{d x} [9 x^{3}]$ .

Since $9$ is constant with respect to $x$ , the derivative of $9 x^{3}$ with respect to $x$ is $9 \frac{d}{d x} [x^{3}]$ .

$9 \frac{d}{d x} [x^{3}] + \frac{d}{d x} [6 x^{2}] + \frac{d}{d x} [- 7]$

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

$9 (3 x^{2}) + \frac{d}{d x} [6 x^{2}] + \frac{d}{d x} [- 7]$

Multiply $3$ by $9$ .

$27 x^{2} + \frac{d}{d x} [6 x^{2}] + \frac{d}{d x} [- 7]$

Evaluate $\frac{d}{d x} [6 x^{2}]$ .

Since $6$ is constant with respect to $x$ , the derivative of $6 x^{2}$ with respect to $x$ is $6 \frac{d}{d x} [x^{2}]$ .

$27 x^{2} + 6 \frac{d}{d x} [x^{2}] + \frac{d}{d x} [- 7]$

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

$27 x^{2} + 6 (2 x) + \frac{d}{d x} [- 7]$

Multiply $2$ by $6$ .

$27 x^{2} + 12 x + \frac{d}{d x} [- 7]$

Differentiate using the Constant Rule.

Since $- 7$ is constant with respect to $x$ , the derivative of $- 7$ with respect to $x$ is $0$ .

$27 x^{2} + 12 x + 0$

Add $27 x^{2} + 12 x$ and $0$ .

$27 x^{2} + 12 x$

Step 2

Find the second derivative of the function.

By the Sum Rule, the derivative of $27 x^{2} + 12 x$ with respect to $x$ is $\frac{d}{d x} [27 x^{2}] + \frac{d}{d x} [12 x]$ .

$f'' (x) = \frac{d}{d x} (27 x^{2}) + \frac{d}{d x} (12 x)$

Evaluate $\frac{d}{d x} [27 x^{2}]$ .

Since $27$ is constant with respect to $x$ , the derivative of $27 x^{2}$ with respect to $x$ is $27 \frac{d}{d x} [x^{2}]$ .

$f'' (x) = 27 \frac{d}{d x} (x^{2}) + \frac{d}{d x} (12 x)$

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

$f'' (x) = 27 (2 x) + \frac{d}{d x} (12 x)$

Multiply $2$ by $27$ .

$f'' (x) = 54 x + \frac{d}{d x} (12 x)$

Evaluate $\frac{d}{d x} [12 x]$ .

Since $12$ is constant with respect to $x$ , the derivative of $12 x$ with respect to $x$ is $12 \frac{d}{d x} [x]$ .

$f'' (x) = 54 x + 12 \frac{d}{d x} (x)$

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

$f'' (x) = 54 x + 12 \cdot 1$

Multiply $12$ by $1$ .

$f'' (x) = 54 x + 12$

Step 3

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

$27 x^{2} + 12 x = 0$

Step 4

Find the first derivative.

By the Sum Rule, the derivative of $9 x^{3} + 6 x^{2} - 7$ with respect to $x$ is $\frac{d}{d x} [9 x^{3}] + \frac{d}{d x} [6 x^{2}] + \frac{d}{d x} [- 7]$ .

$f' (x) = \frac{d}{d x} (9 x^{3}) + \frac{d}{d x} (6 x^{2}) + \frac{d}{d x} (- 7)$

Evaluate $\frac{d}{d x} [9 x^{3}]$ .

Since $9$ is constant with respect to $x$ , the derivative of $9 x^{3}$ with respect to $x$ is $9 \frac{d}{d x} [x^{3}]$ .

$f' (x) = 9 \frac{d}{d x} (x^{3}) + \frac{d}{d x} (6 x^{2}) + \frac{d}{d x} (- 7)$

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

$f' (x) = 9 (3 x^{2}) + \frac{d}{d x} (6 x^{2}) + \frac{d}{d x} (- 7)$

Multiply $3$ by $9$ .

$f' (x) = 27 x^{2} + \frac{d}{d x} (6 x^{2}) + \frac{d}{d x} (- 7)$

Evaluate $\frac{d}{d x} [6 x^{2}]$ .

Since $6$ is constant with respect to $x$ , the derivative of $6 x^{2}$ with respect to $x$ is $6 \frac{d}{d x} [x^{2}]$ .

$f' (x) = 27 x^{2} + 6 \frac{d}{d x} (x^{2}) + \frac{d}{d x} (- 7)$

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

$f' (x) = 27 x^{2} + 6 (2 x) + \frac{d}{d x} (- 7)$

Multiply $2$ by $6$ .

$f' (x) = 27 x^{2} + 12 x + \frac{d}{d x} (- 7)$

Differentiate using the Constant Rule.

Since $- 7$ is constant with respect to $x$ , the derivative of $- 7$ with respect to $x$ is $0$ .

$f' (x) = 27 x^{2} + 12 x + 0$

Add $27 x^{2} + 12 x$ and $0$ .

$f' (x) = 27 x^{2} + 12 x$

The first derivative of $f (x)$ with respect to $x$ is $27 x^{2} + 12 x$ .

$27 x^{2} + 12 x$

Step 5

Set the first derivative equal to $0$ then solve the equation $27 x^{2} + 12 x = 0$ .

Set the first derivative equal to $0$ .

$27 x^{2} + 12 x = 0$

Factor $3 x$ out of $27 x^{2} + 12 x$ .

Factor $3 x$ out of $27 x^{2}$ .

$3 x (9 x) + 12 x = 0$

Factor $3 x$ out of $12 x$ .

$3 x (9 x) + 3 x (4) = 0$

Factor $3 x$ out of $3 x (9 x) + 3 x (4)$ .

$3 x (9 x + 4) = 0$

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

$x = 0$

$9 x + 4 = 0$

Set $x$ equal to $0$ .

$x = 0$

Set $9 x + 4$ equal to $0$ and solve for $x$ .

Set $9 x + 4$ equal to $0$ .

$9 x + 4 = 0$

Solve $9 x + 4 = 0$ for $x$ .

Subtract $4$ from both sides of the equation.

$9 x = - 4$

Divide each term in $9 x = - 4$ by $9$ and simplify.

Divide each term in $9 x = - 4$ by $9$ .

$\frac{9 x}{9} = \frac{- 4}{9}$

Simplify the left side.

Cancel the common factor of $9$ .

Cancel the common factor.

$\frac{9 x}{9} = \frac{- 4}{9}$

Divide $x$ by $1$ .

$x = \frac{- 4}{9}$

Simplify the right side.

Move the negative in front of the fraction.

$x = - \frac{4}{9}$

The final solution is all the values that make $3 x (9 x + 4) = 0$ true.

$x = 0, - \frac{4}{9}$

Step 6

Find the values where the derivative is undefined.

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 7

Critical points to evaluate.

$x = 0, - \frac{4}{9}$

Step 8

Evaluate the second derivative at $x = 0$ . If the second derivative is positive, then this is a local minimum. If it is negative, then this is a local maximum.

$54 (0) + 12$

Step 9

Evaluate the second derivative.

Multiply $54$ by $0$ .

$0 + 12$

Add $0$ and $12$ .

$12$

Step 10

$x = 0$ is a local minimum because the value of the second derivative is positive. This is referred to as the second derivative test.

$x = 0$ is a local minimum

Step 11

Find the y-value when $x = 0$ .

Replace the variable $x$ with $0$ in the expression.

$f (0) = 9 {(0)}^{3} + 6 {(0)}^{2} - 7$

Simplify the result.

Simplify each term.

Raising $0$ to any positive power yields $0$ .

$f (0) = 9 \cdot 0 + 6 {(0)}^{2} - 7$

Multiply $9$ by $0$ .

$f (0) = 0 + 6 {(0)}^{2} - 7$

Raising $0$ to any positive power yields $0$ .

$f (0) = 0 + 6 \cdot 0 - 7$

Multiply $6$ by $0$ .

$f (0) = 0 + 0 - 7$

Simplify by adding and subtracting.

Add $0$ and $0$ .

$f (0) = 0 - 7$

Subtract $7$ from $0$ .

$f (0) = - 7$

The final answer is $- 7$ .

$y = - 7$

Step 12

Evaluate the second derivative at $x = - \frac{4}{9}$ . If the second derivative is positive, then this is a local minimum. If it is negative, then this is a local maximum.

$54 (- \frac{4}{9}) + 12$

Step 13

Evaluate the second derivative.

Simplify each term.

Cancel the common factor of $9$ .

Move the leading negative in $- \frac{4}{9}$ into the numerator.

$54 (\frac{- 4}{9}) + 12$

Factor $9$ out of $54$ .

$9 (6) \frac{- 4}{9} + 12$

Cancel the common factor.

$9 \cdot 6 \frac{- 4}{9} + 12$

Rewrite the expression.

$6 \cdot - 4 + 12$

Multiply $6$ by $- 4$ .

$- 24 + 12$

Add $- 24$ and $12$ .

$- 12$

Step 14

$x = - \frac{4}{9}$ is a local maximum because the value of the second derivative is negative. This is referred to as the second derivative test.

$x = - \frac{4}{9}$ is a local maximum

Step 15

Find the y-value when $x = - \frac{4}{9}$ .

Replace the variable $x$ with $- \frac{4}{9}$ in the expression.

$f (- \frac{4}{9}) = 9 {(- \frac{4}{9})}^{3} + 6 {(- \frac{4}{9})}^{2} - 7$

Simplify the result.

Simplify each term.

Use the power rule ${(a b)}^{n} = a^{n} b^{n}$ to distribute the exponent.

Apply the product rule to $- \frac{4}{9}$ .

$f (- \frac{4}{9}) = 9 ({(- 1)}^{3} {(\frac{4}{9})}^{3}) + 6 {(- \frac{4}{9})}^{2} - 7$

Apply the product rule to $\frac{4}{9}$ .

$f (- \frac{4}{9}) = 9 ({(- 1)}^{3} (\frac{4^{3}}{9^{3}})) + 6 {(- \frac{4}{9})}^{2} - 7$

Raise $- 1$ to the power of $3$ .

$f (- \frac{4}{9}) = 9 (- \frac{4^{3}}{9^{3}}) + 6 {(- \frac{4}{9})}^{2} - 7$

Raise $4$ to the power of $3$ .

$f (- \frac{4}{9}) = 9 (- \frac{64}{9^{3}}) + 6 {(- \frac{4}{9})}^{2} - 7$

Raise $9$ to the power of $3$ .

$f (- \frac{4}{9}) = 9 (- \frac{64}{729}) + 6 {(- \frac{4}{9})}^{2} - 7$

Cancel the common factor of $9$ .

Move the leading negative in $- \frac{64}{729}$ into the numerator.

$f (- \frac{4}{9}) = 9 (\frac{- 64}{729}) + 6 {(- \frac{4}{9})}^{2} - 7$

Factor $9$ out of $729$ .

$f (- \frac{4}{9}) = 9 (\frac{- 64}{9 (81)}) + 6 {(- \frac{4}{9})}^{2} - 7$

Cancel the common factor.

$f (- \frac{4}{9}) = 9 (\frac{- 64}{9 \cdot 81}) + 6 {(- \frac{4}{9})}^{2} - 7$

Rewrite the expression.

$f (- \frac{4}{9}) = \frac{- 64}{81} + 6 {(- \frac{4}{9})}^{2} - 7$

Move the negative in front of the fraction.

$f (- \frac{4}{9}) = - \frac{64}{81} + 6 {(- \frac{4}{9})}^{2} - 7$

Use the power rule ${(a b)}^{n} = a^{n} b^{n}$ to distribute the exponent.

Apply the product rule to $- \frac{4}{9}$ .

$f (- \frac{4}{9}) = - \frac{64}{81} + 6 ({(- 1)}^{2} {(\frac{4}{9})}^{2}) - 7$

Apply the product rule to $\frac{4}{9}$ .

$f (- \frac{4}{9}) = - \frac{64}{81} + 6 ({(- 1)}^{2} (\frac{4^{2}}{9^{2}})) - 7$

Raise $- 1$ to the power of $2$ .

$f (- \frac{4}{9}) = - \frac{64}{81} + 6 (1 (\frac{4^{2}}{9^{2}})) - 7$

Multiply $\frac{4^{2}}{9^{2}}$ by $1$ .

$f (- \frac{4}{9}) = - \frac{64}{81} + 6 (\frac{4^{2}}{9^{2}}) - 7$

Raise $4$ to the power of $2$ .

$f (- \frac{4}{9}) = - \frac{64}{81} + 6 (\frac{16}{9^{2}}) - 7$

Raise $9$ to the power of $2$ .

$f (- \frac{4}{9}) = - \frac{64}{81} + 6 (\frac{16}{81}) - 7$

Cancel the common factor of $3$ .

Factor $3$ out of $6$ .

$f (- \frac{4}{9}) = - \frac{64}{81} + 3 (2) (\frac{16}{81}) - 7$

Factor $3$ out of $81$ .

$f (- \frac{4}{9}) = - \frac{64}{81} + 3 \cdot (2 (\frac{16}{3 \cdot 27})) - 7$

Cancel the common factor.

$f (- \frac{4}{9}) = - \frac{64}{81} + 3 \cdot (2 (\frac{16}{3 \cdot 27})) - 7$

Rewrite the expression.

$f (- \frac{4}{9}) = - \frac{64}{81} + 2 (\frac{16}{27}) - 7$

Combine $2$ and $\frac{16}{27}$ .

$f (- \frac{4}{9}) = - \frac{64}{81} + \frac{2 \cdot 16}{27} - 7$

Multiply $2$ by $16$ .

$f (- \frac{4}{9}) = - \frac{64}{81} + \frac{32}{27} - 7$

Find the common denominator.

Multiply $\frac{32}{27}$ by $\frac{3}{3}$ .

$f (- \frac{4}{9}) = - \frac{64}{81} + \frac{32}{27} \cdot \frac{3}{3} - 7$

Multiply $\frac{32}{27}$ by $\frac{3}{3}$ .

$f (- \frac{4}{9}) = - \frac{64}{81} + \frac{32 \cdot 3}{27 \cdot 3} - 7$

Write $- 7$ as a fraction with denominator $1$ .

$f (- \frac{4}{9}) = - \frac{64}{81} + \frac{32 \cdot 3}{27 \cdot 3} + \frac{- 7}{1}$

Multiply $\frac{- 7}{1}$ by $\frac{81}{81}$ .

$f (- \frac{4}{9}) = - \frac{64}{81} + \frac{32 \cdot 3}{27 \cdot 3} + \frac{- 7}{1} \cdot \frac{81}{81}$

Multiply $\frac{- 7}{1}$ by $\frac{81}{81}$ .

$f (- \frac{4}{9}) = - \frac{64}{81} + \frac{32 \cdot 3}{27 \cdot 3} + \frac{- 7 \cdot 81}{81}$

Reorder the factors of $27 \cdot 3$ .

$f (- \frac{4}{9}) = - \frac{64}{81} + \frac{32 \cdot 3}{3 \cdot 27} + \frac{- 7 \cdot 81}{81}$

Multiply $3$ by $27$ .

$f (- \frac{4}{9}) = - \frac{64}{81} + \frac{32 \cdot 3}{81} + \frac{- 7 \cdot 81}{81}$

Combine the numerators over the common denominator.

$f (- \frac{4}{9}) = \frac{- 64 + 32 \cdot 3 - 7 \cdot 81}{81}$

Simplify each term.

Multiply $32$ by $3$ .

$f (- \frac{4}{9}) = \frac{- 64 + 96 - 7 \cdot 81}{81}$

Multiply $- 7$ by $81$ .

$f (- \frac{4}{9}) = \frac{- 64 + 96 - 567}{81}$

Simplify the expression.

Add $- 64$ and $96$ .

$f (- \frac{4}{9}) = \frac{32 - 567}{81}$

Subtract $567$ from $32$ .

$f (- \frac{4}{9}) = \frac{- 535}{81}$

Move the negative in front of the fraction.

$f (- \frac{4}{9}) = - \frac{535}{81}$

The final answer is $- \frac{535}{81}$ .

$y = - \frac{535}{81}$

Step 16

These are the local extrema for $f (x) = 9 x^{3} + 6 x^{2} - 7$ .

$(0, - 7)$ is a local minima

$(- \frac{4}{9}, - \frac{535}{81})$ is a local maxima

Step 17

Find the Maximum/Minimum Value f(x)=9x^3+6x^2-7

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