# Inflexion points (1 Viewer)

#### Joshmosh2

##### Member
If I find the second derivative for a function, and end up with something like
y'' = 6x -7
How can I find out the points of inflexion?
Also, how can I determine whether they are concave up or down?

For finding the points, I am assuming y'' = 0 and solve for x
and from there, depending on whether the the solution for x is positive or negative, that will determine concave up or down?
Not entirely sure.
Thanks.

#### randomly_generated

##### Member
Yes for possible points of inflection, solve second diff = 0 and then determine concavity

#### Joshmosh2

##### Member
Yes for possible points of inflection, solve second diff = 0 and then determine concavity
thanks, and also, to find the y-coordinate, sub into f(x) or f'(x)?

#### Stygian

##### Active Member
for the y coordinate sub into f(x)

#### randomly_generated

##### Member
Subbing x into y' is meaningless in hsc maths

#### ConorDevaney

##### Member
when proving it a point of inflexion, you have to test for values above and below the "possible point go inflexion", otherwise you won't receive the full marks of the question