I have a differentiation problem which starts with

I have to find dy/dx. No problem there, get

Then to find values of x where dy/dy = 0

But then it asks "Denoting the values of x which you have just calculated by a and b, where a < b, show dy/dx is positive when a < x < b".

I can see graphically where dy/dx is positive, but do I need to show algebraically? How would I answer this?

Angus