Second Derivative Calculator
Calculate the second derivative of a function for concavity and inflection point analysis. Enter values for instant results with step-by-step formulas.
Formula
f''(x) = d/dx[f'(x)] = d^2f/dx^2
The second derivative is found by differentiating the first derivative. For polynomials, apply the power rule twice. Concave up when f''(x) > 0, concave down when f''(x) < 0. Inflection points occur where f''(x) = 0 and concavity changes.
Worked Examples
Example 1: Cubic Polynomial Concavity Analysis
Problem: Find the second derivative, concavity, and inflection point for f(x) = 2x^3 - 6x^2 + 4x - 1 at x = 3.
Solution: f(x) = 2x^3 - 6x^2 + 4x - 1\nf'(x) = 6x^2 - 12x + 4\nf''(x) = 12x - 12\nf''(3) = 12(3) - 12 = 36 - 12 = 24\nSince f''(3) > 0, the function is concave up at x = 3\nInflection point: f''(x) = 0 => 12x - 12 = 0 => x = 1\nf(1) = 2 - 6 + 4 - 1 = -1, so inflection at (1, -1)
Result: f''(3) = 24 (Concave Up) | Inflection Point: (1, -1)
Example 2: Second Derivative Test for Extrema
Problem: For f(x) = x^3 - 3x^2 + 1, find and classify all critical points using the second derivative test.
Solution: f'(x) = 3x^2 - 6x = 3x(x - 2)\nCritical points: x = 0 and x = 2\nf''(x) = 6x - 6\nf''(0) = -6 < 0, so x = 0 is a local maximum, f(0) = 1\nf''(2) = 6 > 0, so x = 2 is a local minimum, f(2) = -3\nInflection point: f''(x) = 0 => x = 1
Result: Local max at (0, 1) | Local min at (2, -3) | Inflection at x = 1
Frequently Asked Questions
What is the second derivative and what does it tell you?
The second derivative is the derivative of the first derivative, representing the rate of change of the rate of change of a function. While the first derivative tells you the slope or velocity, the second derivative tells you how that slope is changing, which corresponds to acceleration in physics. A positive second derivative means the function is concave up (shaped like a cup), while a negative second derivative means the function is concave down (shaped like a cap). The second derivative is essential for determining whether critical points found by the first derivative are local maxima, local minima, or neither.
How do you find the second derivative of a polynomial?
To find the second derivative of a polynomial, apply the power rule twice. The power rule states that the derivative of ax^n is n times a times x^(n-1). For example, for f(x) = 3x^4 + 2x^3 - 5x^2 + x - 7, the first derivative is 12x^3 + 6x^2 - 10x + 1, and the second derivative is 36x^2 + 12x - 10. Each differentiation reduces the power of each term by one, so a cubic polynomial produces a linear second derivative and a quadratic polynomial produces a constant second derivative. Terms with x^1 disappear after the second derivative, and constant terms vanish after the first.
What are inflection points and how does the second derivative find them?
An inflection point is where a function changes its concavity, transitioning from concave up to concave down or vice versa. To find inflection points, set the second derivative equal to zero and solve for x, then verify that the concavity actually changes at those points. A zero second derivative is necessary but not sufficient for an inflection point because the concavity must actually switch sign. For example, f(x) = x^4 has a second derivative of 12x^2, which equals zero at x = 0, but there is no inflection point because the second derivative is positive on both sides. Inflection points are important in economics for identifying points of diminishing returns.
What is the second derivative test for local extrema?
The second derivative test is a method to classify critical points as local maxima or minima. First, find the critical points by setting the first derivative equal to zero. Then evaluate the second derivative at each critical point. If the second derivative is positive at that point, the function is concave up and the critical point is a local minimum. If the second derivative is negative, the function is concave down and the critical point is a local maximum. If the second derivative equals zero, the test is inconclusive and you must use the first derivative test instead. This test is faster than the first derivative test when the second derivative is easy to compute.
How does the second derivative relate to acceleration in physics?
In physics, if position is described by s(t), the first derivative s'(t) gives velocity, and the second derivative s''(t) gives acceleration. This is because velocity measures how position changes over time, and acceleration measures how velocity changes over time. Positive acceleration means the object is speeding up (if moving forward) or slowing down (if moving backward), while negative acceleration means the opposite. For example, an object in freefall has a position function s(t) = -4.9t^2 + v0t + s0, a velocity function s'(t) = -9.8t + v0, and a constant acceleration of s''(t) = -9.8 m/s^2, which is gravitational acceleration.
What is the second derivative of trigonometric functions?
The second derivatives of trigonometric functions follow cyclic patterns. The second derivative of sin(x) is -sin(x), and the second derivative of cos(x) is -cos(x). This means that differentiating sin(x) four times returns to sin(x), creating a cycle of period 4. For the general form a*sin(bx + c), the first derivative is a*b*cos(bx + c), and the second derivative is -a*b^2*sin(bx + c). This negative relationship between a trig function and its second derivative is the mathematical basis for simple harmonic motion, wave equations, and oscillatory systems in physics and engineering.