The Student Room Group

Points of inflexion and stationary points.

I am finding the point of inflexion by rearranging the d^2y/dx^2 = 0 to find x and then plugging the number into the original equation to find y which gives me one set of co-ordinates for the point of inflexion.

But I think sometimes there can be more than one point of inflexion, how do I find others please?
Original post by jami74
I am finding the point
of inflexion by rearranging the d^2y/dx^2 = 0 to
find x and then plugging the number into the
original equation to find y which gives me one set
of co-ordinates for the point of inflexion.
But I think sometimes there can be more than
one point of inflexion, how do I find others please?


Then when you set dy/dx or d2y/dx2d^2y/dx^2 equal to zero, you'll get a quadratic or qubic or quartic which you can then solve.
Original post by jami74
I am finding the point of inflexion by rearranging the d^2y/dx^2 = 0 to find x and then plugging the number into the original equation to find y which gives me one set of co-ordinates for the point of inflexion.

But I think sometimes there can be more than one point of inflexion, how do I find others please?


*Inflection & if the equation d2y/dx2 = 0 has one solution, then there is one point of inflection, if it has 3 solutions then it has 3 points of inflection etc.
Reply 3
Thank-you both for answering.

Original post by This Excellency
Then when you set dy/dx or d2y/dx2d^2y/dx^2 equal to zero, you'll get a quadratic or qubic or quartic which you can then solve.


Original post by marcus2001
*Inflection & if the equation d2y/dx2 = 0 has one solution, then there is one point of inflection, if it has 3 solutions then it has 3 points of inflection etc.


My d^2y/dx^2 was giving things like 6x-2
Original post by jami74
Thank-you both for answering.





My d^2y/dx^2 was giving things like 6x-2


That's okay, well that clearly has only one solution, x=1/3 so one inflection point. If the function was something different like a higher order polynomial, trig function or something else, there could be more solutions though and more inflection points.
Reply 5
Original post by jami74
I am finding the point of inflexion by rearranging the d^2y/dx^2 = 0 to find x and then plugging the number into the original equation to find y which gives me one set of co-ordinates for the point of inflexion.

But I think sometimes there can be more than one point of inflexion, how do I find others please?


Note that d2ydx2=0\frac{d^2y}{dx^2} = 0 isn't a sufficient condition for a point of inflexion - look at the graph of y=x4y=x^4 for a counter-example.

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