Find the maximum and minimum values of f(x) = 3x³ - 9x² - 27x + 15.
To find the maximum and minimum values of the function f(x) = 3x³ - 9x² - 27x + 15, we need to find the critical points by taking the derivative of the function and setting it equal to zero.
1. Find the first derivative:
f'(x) = d/dx (3x³ - 9x² - 27x + 15)
f'(x) = 9x² - 18x - 27
2. Set the first derivative equal to zero and solve for x:
9x² - 18x - 27 = 0
Divide by 9: x² - 2x - 3 = 0
Factor: (x - 3)(x + 1) = 0
So, x = 3 or x = -1
3. Find the second derivative:
f''(x) = d/dx (9x² - 18x - 27)
f''(x) = 18x - 18
4. Use the second derivative test to determine if the critical points are maximum or minimum:
Evaluate f''(x) at x = 3:
f''(3) = 18(3) - 18 = 54 - 18 = 36
Since f''(3) > 0, x = 3 is a local minimum.
Evaluate f''(x) at x = -1:
f''(-1) = 18(-1) - 18 = -18 - 18 = -36
Since f''(-1) < 0, x = -1 is a local maximum.
5. Find the values of the function at the critical points:
f(3) = 3(3)³ - 9(3)² - 27(3) + 15 = 3(27) - 9(9) - 81 + 15 = 81 - 81 - 81 + 15 = -66
f(-1) = 3(-1)³ - 9(-1)² - 27(-1) + 15 = 3(-1) - 9(1) + 27 + 15 = -3 - 9 + 27 + 15 = 30
6. Determine the maximum and minimum values:
The local maximum value is 30 at x = -1.
The local minimum value is -66 at x = 3.
In summary:
- Local Maximum: 30 at x = -1
- Local Minimum: -66 at x = 3