Find the Maximum/Minimum Value f(x)=x^2+15x-x^5

$f (x) = x^{2} + 15 x - x^{5}$

Step 1

Find the first derivative of the function.

Differentiate.

By the Sum Rule, the derivative of $x^{2} + 15 x - x^{5}$ with respect to $x$ is $\frac{d}{d x} [x^{2}] + \frac{d}{d x} [15 x] + \frac{d}{d x} [- x^{5}]$ .

$\frac{d}{d x} [x^{2}] + \frac{d}{d x} [15 x] + \frac{d}{d x} [- x^{5}]$

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

$2 x + \frac{d}{d x} [15 x] + \frac{d}{d x} [- x^{5}]$

Evaluate $\frac{d}{d x} [15 x]$ .

Since $15$ is constant with respect to $x$ , the derivative of $15 x$ with respect to $x$ is $15 \frac{d}{d x} [x]$ .

$2 x + 15 \frac{d}{d x} [x] + \frac{d}{d x} [- x^{5}]$

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

$2 x + 15 \cdot 1 + \frac{d}{d x} [- x^{5}]$

Multiply $15$ by $1$ .

$2 x + 15 + \frac{d}{d x} [- x^{5}]$

Evaluate $\frac{d}{d x} [- x^{5}]$ .

Since $- 1$ is constant with respect to $x$ , the derivative of $- x^{5}$ with respect to $x$ is $- \frac{d}{d x} [x^{5}]$ .

$2 x + 15 - \frac{d}{d x} [x^{5}]$

Differentiate using the Power Rule which states that $\frac{d}{d x} [x^{n}]$ is $n x^{n - 1}$ where $n = 5$ .

$2 x + 15 - (5 x^{4})$

Multiply $5$ by $- 1$ .

$2 x + 15 - 5 x^{4}$

Reorder terms.

$- 5 x^{4} + 2 x + 15$

Step 2

Find the second derivative of the function.

By the Sum Rule, the derivative of $- 5 x^{4} + 2 x + 15$ with respect to $x$ is $\frac{d}{d x} [- 5 x^{4}] + \frac{d}{d x} [2 x] + \frac{d}{d x} [15]$ .

$f'' (x) = \frac{d}{d x} (- 5 x^{4}) + \frac{d}{d x} (2 x) + \frac{d}{d x} (15)$

Evaluate $\frac{d}{d x} [- 5 x^{4}]$ .

Since $- 5$ is constant with respect to $x$ , the derivative of $- 5 x^{4}$ with respect to $x$ is $- 5 \frac{d}{d x} [x^{4}]$ .

$f'' (x) = - 5 \frac{d}{d x} x^{4} + \frac{d}{d x} (2 x) + \frac{d}{d x} (15)$

Differentiate using the Power Rule which states that $\frac{d}{d x} [x^{n}]$ is $n x^{n - 1}$ where $n = 4$ .

$f'' (x) = - 5 (4 x^{3}) + \frac{d}{d x} (2 x) + \frac{d}{d x} (15)$

Multiply $4$ by $- 5$ .

$f'' (x) = - 20 x^{3} + \frac{d}{d x} (2 x) + \frac{d}{d x} (15)$

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

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

$f'' (x) = - 20 x^{3} + 2 \frac{d}{d x} (x) + \frac{d}{d x} (15)$

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

$f'' (x) = - 20 x^{3} + 2 \cdot 1 + \frac{d}{d x} (15)$

Multiply $2$ by $1$ .

$f'' (x) = - 20 x^{3} + 2 + \frac{d}{d x} (15)$

Differentiate using the Constant Rule.

Since $15$ is constant with respect to $x$ , the derivative of $15$ with respect to $x$ is $0$ .

$f'' (x) = - 20 x^{3} + 2 + 0$

Add $- 20 x^{3} + 2$ and $0$ .

$f'' (x) = - 20 x^{3} + 2$

Step 3

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

$- 5 x^{4} + 2 x + 15 = 0$

Step 4

Find the first derivative.

Differentiate.

By the Sum Rule, the derivative of $x^{2} + 15 x - x^{5}$ with respect to $x$ is $\frac{d}{d x} [x^{2}] + \frac{d}{d x} [15 x] + \frac{d}{d x} [- x^{5}]$ .

$f' (x) = \frac{d}{d x} (x^{2}) + \frac{d}{d x} (15 x) + \frac{d}{d x} (- x^{5})$

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

$f' (x) = 2 x + \frac{d}{d x} (15 x) + \frac{d}{d x} (- x^{5})$

Evaluate $\frac{d}{d x} [15 x]$ .

Since $15$ is constant with respect to $x$ , the derivative of $15 x$ with respect to $x$ is $15 \frac{d}{d x} [x]$ .

$f' (x) = 2 x + 15 \frac{d}{d x} (x) + \frac{d}{d x} (- x^{5})$

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

$f' (x) = 2 x + 15 \cdot 1 + \frac{d}{d x} (- x^{5})$

Multiply $15$ by $1$ .

$f' (x) = 2 x + 15 + \frac{d}{d x} (- x^{5})$

Evaluate $\frac{d}{d x} [- x^{5}]$ .

Since $- 1$ is constant with respect to $x$ , the derivative of $- x^{5}$ with respect to $x$ is $- \frac{d}{d x} [x^{5}]$ .

$f' (x) = 2 x + 15 - \frac{d}{d x} x^{5}$

Differentiate using the Power Rule which states that $\frac{d}{d x} [x^{n}]$ is $n x^{n - 1}$ where $n = 5$ .

$f' (x) = 2 x + 15 - (5 x^{4})$

Multiply $5$ by $- 1$ .

$f' (x) = 2 x + 15 - 5 x^{4}$

Reorder terms.

$f' (x) = - 5 x^{4} + 2 x + 15$

The first derivative of $f (x)$ with respect to $x$ is $- 5 x^{4} + 2 x + 15$ .

$- 5 x^{4} + 2 x + 15$

Step 5

Set the first derivative equal to $0$ then solve the equation $- 5 x^{4} + 2 x + 15 = 0$ .

Set the first derivative equal to $0$ .

$- 5 x^{4} + 2 x + 15 = 0$

Graph each side of the equation. The solution is the x-value of the point of intersection.

$x \approx - \frac{8 \sqrt{2}}{9}, 1.3725465$

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.

$x = - \frac{8 \sqrt{2}}{9}, 1.3725465$

Step 8

Evaluate the second derivative at $x = - \frac{8 \sqrt{2}}{9}$ . If the second derivative is positive, then this is a local minimum. If it is negative, then this is a local maximum.

$- 20 {(- \frac{8 \sqrt{2}}{9})}^{3} + 2$

Step 9

Simplify each term.

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

Apply the product rule to $- \frac{8 \sqrt{2}}{9}$ .

$- 20 ({(- 1)}^{3} {(\frac{8 \sqrt{2}}{9})}^{3}) + 2$

Apply the product rule to $\frac{8 \sqrt{2}}{9}$ .

$- 20 ({(- 1)}^{3} \frac{{(8 \sqrt{2})}^{3}}{9^{3}}) + 2$

Apply the product rule to $8 \sqrt{2}$ .

$- 20 ({(- 1)}^{3} \frac{8^{3} {\sqrt{2}}^{3}}{9^{3}}) + 2$

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

$- 20 (- \frac{8^{3} {\sqrt{2}}^{3}}{9^{3}}) + 2$

Simplify the numerator.

Raise $8$ to the power of $3$ .

$- 20 (- \frac{512 {\sqrt{2}}^{3}}{9^{3}}) + 2$

Rewrite ${\sqrt{2}}^{3}$ as $\sqrt{2^{3}}$ .

$- 20 (- \frac{512 \sqrt{2^{3}}}{9^{3}}) + 2$

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

$- 20 (- \frac{512 \sqrt{8}}{9^{3}}) + 2$

Rewrite $8$ as $2^{2} \cdot 2$ .

Factor $4$ out of $8$ .

$- 20 (- \frac{512 \sqrt{4 (2)}}{9^{3}}) + 2$

Rewrite $4$ as $2^{2}$ .

$- 20 (- \frac{512 \sqrt{2^{2} \cdot 2}}{9^{3}}) + 2$

Pull terms out from under the radical.

$- 20 (- \frac{512 \cdot 2 \sqrt{2}}{9^{3}}) + 2$

Multiply $512$ by $2$ .

$- 20 (- \frac{1024 \sqrt{2}}{9^{3}}) + 2$

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

$- 20 (- \frac{1024 \sqrt{2}}{729}) + 2$

Multiply $- 20 (- \frac{1024 \sqrt{2}}{729})$ .

Multiply $- 1$ by $- 20$ .

$20 \frac{1024 \sqrt{2}}{729} + 2$

Combine $20$ and $\frac{1024 \sqrt{2}}{729}$ .

$\frac{20 (1024 \sqrt{2})}{729} + 2$

Multiply $1024$ by $20$ .

$\frac{20480 \sqrt{2}}{729} + 2$

Step 10

$x = - \frac{8 \sqrt{2}}{9}$ is a local minimum because the value of the second derivative is positive. This is referred to as the second derivative test.

$x = - \frac{8 \sqrt{2}}{9}$ is a local minimum

Step 11

Find the y-value when $x = - \frac{8 \sqrt{2}}{9}$ .

Replace the variable $x$ with $- \frac{8 \sqrt{2}}{9}$ in the expression.

$f (- \frac{8 \sqrt{2}}{9}) = {(- \frac{8 \sqrt{2}}{9})}^{2} + 15 (- \frac{8 \sqrt{2}}{9}) - {(- \frac{8 \sqrt{2}}{9})}^{5}$

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 $- \frac{8 \sqrt{2}}{9}$ .

$f (- \frac{8 \sqrt{2}}{9}) = {(- 1)}^{2} {(\frac{8 \sqrt{2}}{9})}^{2} + 15 (- \frac{8 \sqrt{2}}{9}) - {(- \frac{8 \sqrt{2}}{9})}^{5}$

Apply the product rule to $\frac{8 \sqrt{2}}{9}$ .

$f (- \frac{8 \sqrt{2}}{9}) = {(- 1)}^{2} (\frac{{(8 \sqrt{2})}^{2}}{9^{2}}) + 15 (- \frac{8 \sqrt{2}}{9}) - {(- \frac{8 \sqrt{2}}{9})}^{5}$

Apply the product rule to $8 \sqrt{2}$ .

$f (- \frac{8 \sqrt{2}}{9}) = {(- 1)}^{2} (\frac{8^{2} {\sqrt{2}}^{2}}{9^{2}}) + 15 (- \frac{8 \sqrt{2}}{9}) - {(- \frac{8 \sqrt{2}}{9})}^{5}$

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

$f (- \frac{8 \sqrt{2}}{9}) = 1 (\frac{8^{2} {\sqrt{2}}^{2}}{9^{2}}) + 15 (- \frac{8 \sqrt{2}}{9}) - {(- \frac{8 \sqrt{2}}{9})}^{5}$

Multiply $\frac{8^{2} {\sqrt{2}}^{2}}{9^{2}}$ by $1$ .

$f (- \frac{8 \sqrt{2}}{9}) = \frac{8^{2} {\sqrt{2}}^{2}}{9^{2}} + 15 (- \frac{8 \sqrt{2}}{9}) - {(- \frac{8 \sqrt{2}}{9})}^{5}$

Simplify the numerator.

Raise $8$ to the power of $2$ .

$f (- \frac{8 \sqrt{2}}{9}) = \frac{64 {\sqrt{2}}^{2}}{9^{2}} + 15 (- \frac{8 \sqrt{2}}{9}) - {(- \frac{8 \sqrt{2}}{9})}^{5}$

Rewrite ${\sqrt{2}}^{2}$ as $2$ .

Use $\sqrt[n]{a^{x}} = a^{\frac{x}{n}}$ to rewrite $\sqrt{2}$ as $2^{\frac{1}{2}}$ .

$f (- \frac{8 \sqrt{2}}{9}) = \frac{64 {(2^{\frac{1}{2}})}^{2}}{9^{2}} + 15 (- \frac{8 \sqrt{2}}{9}) - {(- \frac{8 \sqrt{2}}{9})}^{5}$

Apply the power rule and multiply exponents, ${(a^{m})}^{n} = a^{m n}$ .

$f (- \frac{8 \sqrt{2}}{9}) = \frac{64 \cdot 2^{\frac{1}{2} \cdot 2}}{9^{2}} + 15 (- \frac{8 \sqrt{2}}{9}) - {(- \frac{8 \sqrt{2}}{9})}^{5}$

Combine $\frac{1}{2}$ and $2$ .

$f (- \frac{8 \sqrt{2}}{9}) = \frac{64 \cdot 2^{\frac{2}{2}}}{9^{2}} + 15 (- \frac{8 \sqrt{2}}{9}) - {(- \frac{8 \sqrt{2}}{9})}^{5}$

Cancel the common factor of $2$ .

Cancel the common factor.

$f (- \frac{8 \sqrt{2}}{9}) = \frac{64 \cdot 2^{\frac{2}{2}}}{9^{2}} + 15 (- \frac{8 \sqrt{2}}{9}) - {(- \frac{8 \sqrt{2}}{9})}^{5}$

Rewrite the expression.

$f (- \frac{8 \sqrt{2}}{9}) = \frac{64 \cdot 2}{9^{2}} + 15 (- \frac{8 \sqrt{2}}{9}) - {(- \frac{8 \sqrt{2}}{9})}^{5}$

Evaluate the exponent.

$f (- \frac{8 \sqrt{2}}{9}) = \frac{64 \cdot 2}{9^{2}} + 15 (- \frac{8 \sqrt{2}}{9}) - {(- \frac{8 \sqrt{2}}{9})}^{5}$

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

$f (- \frac{8 \sqrt{2}}{9}) = \frac{64 \cdot 2}{81} + 15 (- \frac{8 \sqrt{2}}{9}) - {(- \frac{8 \sqrt{2}}{9})}^{5}$

Multiply $64$ by $2$ .

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} + 15 (- \frac{8 \sqrt{2}}{9}) - {(- \frac{8 \sqrt{2}}{9})}^{5}$

Cancel the common factor of $3$ .

Move the leading negative in $- \frac{8 \sqrt{2}}{9}$ into the numerator.

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} + 15 (\frac{- 8 \sqrt{2}}{9}) - {(- \frac{8 \sqrt{2}}{9})}^{5}$

Factor $3$ out of $15$ .

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} + 3 (5) (\frac{- 8 \sqrt{2}}{9}) - {(- \frac{8 \sqrt{2}}{9})}^{5}$

Factor $3$ out of $9$ .

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} + 3 \cdot (5 (\frac{- 8 \sqrt{2}}{3 \cdot 3})) - {(- \frac{8 \sqrt{2}}{9})}^{5}$

Cancel the common factor.

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} + 3 \cdot (5 (\frac{- 8 \sqrt{2}}{3 \cdot 3})) - {(- \frac{8 \sqrt{2}}{9})}^{5}$

Rewrite the expression.

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} + 5 (\frac{- 8 \sqrt{2}}{3}) - {(- \frac{8 \sqrt{2}}{9})}^{5}$

Combine $5$ and $\frac{- 8 \sqrt{2}}{3}$ .

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} + \frac{5 (- 8 \sqrt{2})}{3} - {(- \frac{8 \sqrt{2}}{9})}^{5}$

Multiply $- 8$ by $5$ .

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} + \frac{- 40 \sqrt{2}}{3} - {(- \frac{8 \sqrt{2}}{9})}^{5}$

Move the negative in front of the fraction.

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} - \frac{(40) \sqrt{2}}{3} - {(- \frac{8 \sqrt{2}}{9})}^{5}$

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

Apply the product rule to $- \frac{8 \sqrt{2}}{9}$ .

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} - \frac{40 \sqrt{2}}{3} - ({(- 1)}^{5} {(\frac{8 \sqrt{2}}{9})}^{5})$

Apply the product rule to $\frac{8 \sqrt{2}}{9}$ .

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} - \frac{40 \sqrt{2}}{3} - ({(- 1)}^{5} (\frac{{(8 \sqrt{2})}^{5}}{9^{5}}))$

Apply the product rule to $8 \sqrt{2}$ .

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} - \frac{40 \sqrt{2}}{3} - ({(- 1)}^{5} (\frac{8^{5} {\sqrt{2}}^{5}}{9^{5}}))$

Multiply $- 1$ by ${(- 1)}^{5}$ by adding the exponents.

Move ${(- 1)}^{5}$ .

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} - \frac{40 \sqrt{2}}{3} + {(- 1)}^{5} \cdot (- 1 \frac{8^{5} {\sqrt{2}}^{5}}{9^{5}})$

Multiply ${(- 1)}^{5}$ by $- 1$ .

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

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} - \frac{40 \sqrt{2}}{3} + {(- 1)}^{5} \cdot ((- 1) (\frac{8^{5} {\sqrt{2}}^{5}}{9^{5}}))$

Use the power rule $a^{m} a^{n} = a^{m + n}$ to combine exponents.

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} - \frac{40 \sqrt{2}}{3} + {(- 1)}^{5 + 1} (\frac{8^{5} {\sqrt{2}}^{5}}{9^{5}})$

Add $5$ and $1$ .

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} - \frac{40 \sqrt{2}}{3} + {(- 1)}^{6} (\frac{8^{5} {\sqrt{2}}^{5}}{9^{5}})$

Raise $- 1$ to the power of $6$ .

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} - \frac{40 \sqrt{2}}{3} + 1 (\frac{8^{5} {\sqrt{2}}^{5}}{9^{5}})$

Multiply $\frac{8^{5} {\sqrt{2}}^{5}}{9^{5}}$ by $1$ .

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} - \frac{40 \sqrt{2}}{3} + \frac{8^{5} {\sqrt{2}}^{5}}{9^{5}}$

Simplify the numerator.

Raise $8$ to the power of $5$ .

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} - \frac{40 \sqrt{2}}{3} + \frac{32768 {\sqrt{2}}^{5}}{9^{5}}$

Rewrite ${\sqrt{2}}^{5}$ as $\sqrt{2^{5}}$ .

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} - \frac{40 \sqrt{2}}{3} + \frac{32768 \sqrt{2^{5}}}{9^{5}}$

Raise $2$ to the power of $5$ .

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} - \frac{40 \sqrt{2}}{3} + \frac{32768 \sqrt{32}}{9^{5}}$

Rewrite $32$ as $4^{2} \cdot 2$ .

Factor $16$ out of $32$ .

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} - \frac{40 \sqrt{2}}{3} + \frac{32768 \sqrt{16 (2)}}{9^{5}}$

Rewrite $16$ as $4^{2}$ .

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} - \frac{40 \sqrt{2}}{3} + \frac{32768 \sqrt{4^{2} \cdot 2}}{9^{5}}$

Pull terms out from under the radical.

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} - \frac{40 \sqrt{2}}{3} + \frac{32768 \cdot (4 \sqrt{2})}{9^{5}}$

Multiply $32768$ by $4$ .

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} - \frac{40 \sqrt{2}}{3} + \frac{131072 \sqrt{2}}{9^{5}}$

Raise $9$ to the power of $5$ .

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} - \frac{40 \sqrt{2}}{3} + \frac{131072 \sqrt{2}}{59049}$

To write $- \frac{40 \sqrt{2}}{3}$ as a fraction with a common denominator, multiply by $\frac{19683}{19683}$ .

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} - \frac{40 \sqrt{2}}{3} \cdot \frac{19683}{19683} + \frac{131072 \sqrt{2}}{59049}$

Write each expression with a common denominator of $59049$ , by multiplying each by an appropriate factor of $1$ .

Multiply $\frac{40 \sqrt{2}}{3}$ by $\frac{19683}{19683}$ .

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} - \frac{40 \sqrt{2} \cdot 19683}{3 \cdot 19683} + \frac{131072 \sqrt{2}}{59049}$

Multiply $3$ by $19683$ .

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} - \frac{40 \sqrt{2} \cdot 19683}{59049} + \frac{131072 \sqrt{2}}{59049}$

Combine the numerators over the common denominator.

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} + \frac{- 40 \sqrt{2} \cdot 19683 + 131072 \sqrt{2}}{59049}$

Simplify each term.

Simplify the numerator.

Multiply $19683$ by $- 40$ .

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} + \frac{- 787320 \sqrt{2} + 131072 \sqrt{2}}{59049}$

Add $- 787320 \sqrt{2}$ and $131072 \sqrt{2}$ .

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} + \frac{- 656248 \sqrt{2}}{59049}$

Move the negative in front of the fraction.

$f (- \frac{8 \sqrt{2}}{9}) = \frac{128}{81} - \frac{656248 \sqrt{2}}{59049}$

The final answer is $\frac{128}{81} - \frac{656248 \sqrt{2}}{59049}$ .

$y = \frac{128}{81} - \frac{656248 \sqrt{2}}{59049}$

Step 12

Evaluate the second derivative at $x = 1.3725465$ . If the second derivative is positive, then this is a local minimum. If it is negative, then this is a local maximum.

$- 20 {(1.3725465)}^{3} + 2$

Step 13

Evaluate the second derivative.

Simplify each term.

Raise $1.3725465$ to the power of $3$ .

$- 20 \cdot 2.58571828 + 2$

Multiply $- 20$ by $2.58571828$ .

$- 51.71436573 + 2$

Add $- 51.71436573$ and $2$ .

$- 49.71436573$

Step 14

$x = 1.3725465$ is a local maximum because the value of the second derivative is negative. This is referred to as the second derivative test.

$x = 1.3725465$ is a local maximum

Step 15

Find the y-value when $x = 1.3725465$ .

Replace the variable $x$ with $1.3725465$ in the expression.

$f (1.3725465) = {(1.3725465)}^{2} + 15 (1.3725465) - {(1.3725465)}^{5}$

Simplify the result.

Simplify each term.

Raise $1.3725465$ to the power of $2$ .

$f (1.3725465) = 1.88388391 + 15 (1.3725465) - {(1.3725465)}^{5}$

Multiply $15$ by $1.3725465$ .

$f (1.3725465) = 1.88388391 + 20.5881976 - {(1.3725465)}^{5}$

Raise $1.3725465$ to the power of $5$ .

$f (1.3725465) = 1.88388391 + 20.5881976 - 1 \cdot 4.87119308$

Multiply $- 1$ by $4.87119308$ .

$f (1.3725465) = 1.88388391 + 20.5881976 - 4.87119308$

Simplify by adding and subtracting.

Add $1.88388391$ and $20.5881976$ .

$f (1.3725465) = 22.47208152 - 4.87119308$

Subtract $4.87119308$ from $22.47208152$ .

$f (1.3725465) = 17.60088843$

The final answer is $17.60088843$ .

$y = 17.60088843$

Step 16

These are the local extrema for $f (x) = x^{2} + 15 x - x^{5}$ .

$(- \frac{8 \sqrt{2}}{9}, \frac{128}{81} - \frac{656248 \sqrt{2}}{59049})$ is a local minima

$(1.3725465, 17.60088843)$ is a local maxima

Step 17

Find the Maximum/Minimum Value f(x)=x^2+15x-x^5

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