Q. Find the x-coordinates of all relative minima of f(x).f(x)=−3x3−36x2+81x−46Answer: x=
Find Critical Points: To find the relative minima of the function f(x), we need to find the critical points by taking the derivative of f(x) and setting it equal to zero.f(x)=−3x3−36x2+81x−46Let's find the first derivative f′(x).f′(x)=dxd(−3x3−36x2+81x−46)f′(x)=−9x2−72x+81
Solve Quadratic Equation: Now, we need to find the values of x for which f′(x)=0.−9x2−72x+81=0To solve this quadratic equation, we can factor it or use the quadratic formula. Let's try to factor it first.−9(x2+8x−9)=0Now, we look for two numbers that multiply to −9 and add up to 8. The numbers are 9 and −1.−9(x+9)(x−1)=0
Identify Critical Points: We have two potential critical points from the factored equation:x+9=0 or x−1=0Solving these, we get:x=−9 or x=1These are the critical points where the derivative is zero, but we need to determine which of these points are relative minima.
Second Derivative Test: To determine if these critical points are relative minima, we can use the second derivative test. Let's find the second derivative f′′(x). f′′(x)=dxd(−9x2−72x+81)f′′(x)=−18x−72
Evaluate at x=−9: Now, we evaluate the second derivative at the critical points.For x=−9:f′′(−9)=−18(−9)−72f′′(−9)=162−72f′′(−9)=90Since f''(-9) > 0, the function is concave up at x=−9, indicating a relative minimum.
Evaluate at x=1: For x=1: f′′(1)=−18(1)−72 f′′(1)=−18−72 f′′(1)=−90 Since f''(1) < 0, the function is concave down at x=1, indicating a relative maximum, not a minimum.
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