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The Sign of the Second DerivativeSummaryThe second derivative tells us a lot about the qualitative behaviour of the graph of a function. This is especially important at points close to the critical (stationary) points. Critical points occur where the first derivative is 0.
ExampleConsider the graph of f(x) = 3x4 − 8x3 − 90x2. First obtain the first and second derivatives. f '(x) = 12x3 − 24x2 − 180x and f "(x) = 36x2 − 48x − 180. To find any stationary points, solve the equation f '(x) = 0. f '(x) = 0 = 12x3 − 24x2 − 180x = 12x(x2 − 2x − 15) = 12x(x + 3)(x − 5) There are three solutions, x = 0, x = -3 and x = 5. To determine the type of stationary point, calculate the second derivative at each value of x. f "(x) = 36x2 − 48x − 180 so f "(0) = -180, f "(-3) = 288 and f "(5) = 480. We have respectively a local maximum, then two local minima. To determine the y-coordinate of the point, calculate the value of the function for each value of x. f(x) = 3x4 − 8x3 − 90x2 so f(0) = 0, f(-3) = -351 and f(5) = -1375. Hence (0,0) is a local maximum, (-3,-351) is a local minimum and (5,-1375) is a local minimum. To determine any points of inflection, solve the equation f "(x) = 0, and determine the concavity of the graph. f "(x) = 0 = 36x2 − 48x − 180 = 12(3x2 − 4x − 15) = 12(3x + 5)(x − 3). There are two solutions, x = - If x < - To determine the y-coordinate of these points of inflection, calculate the value of the function for each value of x. f(x) = 3x4 − 8x3 − 90x2 so f(-
f "(x) = 12(3x + 5)(x − 3) ExampleConsider the graph of f(x) = First obtain the first and second derivatives. Since To find any stationary points, solve the equation f '(x) = 0. f '(x) = 0 = f "(x) = f(0) = To determine any points of inflection, solve the equation f "(x) = 0. f "(x) = 0 = To determine the concavity of the graph, consider the sign of the second derivative. f "(x) = If we note that the function has no defined value when x = -3 and x = 3, we can sketch its graph.
Exercise<< Maxima and Minima | Sign of the Second Derivative Index | |