be the function of third degree.
Then the second drivative is:
. With f''(x) = 0 you'll get:
Calculate ....... Therefore
2. If the function has 3 zeros you can write f as a productof the linear factors:
The uninteresting summand contains x to the power of 1 or 0.
Then the second derivative is:
With f''(x) = 0 you'll get the given result.