MediumCalculusca-036

Inflection Point of a Cubic

Problem

Find the x-coordinate of the inflection point of f(x) = a*x^3 + b*x^2 + c*x + d.

The second derivative is f''(x) = 6*a*x + 2*b, which vanishes at x = -b / (3*a). Return None when a == 0, since then the function is not a cubic.

Hint

Set the second derivative to zero and solve the resulting linear equation.

case 1

in: [1,-3,0,0]

expected: 1

case 2

in: [2,6,0,0]

expected: -1

case 3

in: [-1,0,0,0]

expected: 0

case 4

in: [0,5,1,0]

expected: null

Starter codePython

def inflection_point(a, b, c, d):
    # Return None when a == 0
    # Your code here
    pass

4 test cases · run your code against them in the editor