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Find the 
x-coordinates of all relative minima of 
f(x).

f(x)=(3)/(4)x^(4)+2x^(3)-36x^(2)+7
Answer: 
x=

Find the x x -coordinates of all relative minima of f(x) f(x) .\newlinef(x)=34x4+2x336x2+7 f(x)=\frac{3}{4} x^{4}+2 x^{3}-36 x^{2}+7 \newlineAnswer: x= x=

Full solution

Q. Find the x x -coordinates of all relative minima of f(x) f(x) .\newlinef(x)=34x4+2x336x2+7 f(x)=\frac{3}{4} x^{4}+2 x^{3}-36 x^{2}+7 \newlineAnswer: x= x=
  1. Find Derivative of f(x)f(x): To find the relative minima of the function f(x)f(x), we need to find the critical points by taking the derivative of f(x)f(x) and setting it equal to zero. Let's start by finding the first derivative f(x)f'(x).
    f(x)=34x4+2x336x2+7f(x) = \frac{3}{4}x^4 + 2x^3 - 36x^2 + 7
    Differentiate f(x)f(x) with respect to xx:
    f(x)=ddx[34x4]+ddx[2x3]ddx[36x2]+ddx[7]f'(x) = \frac{d}{dx}\left[\frac{3}{4}x^4\right] + \frac{d}{dx}\left[2x^3\right] - \frac{d}{dx}\left[36x^2\right] + \frac{d}{dx}\left[7\right]
    f(x)=434x3+32x2236x+0f'(x) = 4\cdot\frac{3}{4}x^3 + 3\cdot2x^2 - 2\cdot36x + 0
    f(x)=3x3+6x272xf'(x) = 3x^3 + 6x^2 - 72x
  2. Find Critical Points: Now we need to find the critical points by setting f(x)f'(x) equal to zero and solving for xx.
    0=3x3+6x272x0 = 3x^3 + 6x^2 - 72x
    We can factor out a common factor of 3x3x:
    0=3x(x2+2x24)0 = 3x(x^2 + 2x - 24)
    Now we factor the quadratic:
    0=3x(x+6)(x4)0 = 3x(x + 6)(x - 4)
    The critical points are x=0x = 0, x=6x = -6, and x=4x = 4.
  3. Find Second Derivative: To determine which of these critical points are relative minima, we need to use the second derivative test. Let's find the second derivative f(x)f''(x).f(x)=ddx[f(x)]f''(x) = \frac{d}{dx}[f'(x)]f(x)=ddx[3x3+6x272x]f''(x) = \frac{d}{dx}[3x^3 + 6x^2 - 72x]f(x)=33x2+26x72f''(x) = 3\cdot 3x^2 + 2\cdot 6x - 72f(x)=9x2+12x72f''(x) = 9x^2 + 12x - 72
  4. Evaluate Concavity at Critical Points: Now we evaluate the second derivative at each critical point to determine the concavity at those points.\newlineFor x=0x = 0:\newlinef(0)=9(0)2+12(0)72f''(0) = 9(0)^2 + 12(0) - 72\newlinef(0)=72f''(0) = -72\newlineSince f''(0) < 0, the function is concave down at x=0x = 0, which means x=0x = 0 is a relative maximum, not a minimum.
  5. Relative Minimum at x=6x = -6: For x=6x = -6:
    f(6)=9(6)2+12(6)72f''(-6) = 9(-6)^2 + 12(-6) - 72
    f(6)=9(36)7272f''(-6) = 9(36) - 72 - 72
    f(6)=3247272f''(-6) = 324 - 72 - 72
    f(6)=180f''(-6) = 180
    Since f''(-6) > 0, the function is concave up at x=6x = -6, which means x=6x = -6 is a relative minimum.
  6. Relative Minimum at x=4x = 4: For x=4x = 4:
    f(4)=9(4)2+12(4)72f''(4) = 9(4)^2 + 12(4) - 72
    f(4)=9(16)+4872f''(4) = 9(16) + 48 - 72
    f(4)=144+4872f''(4) = 144 + 48 - 72
    f(4)=120f''(4) = 120
    Since f''(4) > 0, the function is concave up at x=4x = 4, which means x=4x = 4 is also a relative minimum.

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