1). f(x)=ln(2x^2+1)

Use derivatives to find the x-values of any critical points and inflection points exactly.

critical points (enter as a comma-separated list): x= 0

inflection points (enter as a comma-separated list): x= ???

The critical point is zero, but I'm not sure what the inflection point is. I thought it was also zero, but that is incorrect.

Indeed the only critical point is x=0 and since f'(x) changes sign when passing through x= 0 (from minus to plus) this is a minimum point. Since , check where this 2nd derivative vanishes for POSSIBLE inflexion points, and then check which derivative after the 2nd one is the first one that does NOT vanish at some of these points: if these last derivative is an odd one then you have an inflexion point, othewise you haven't.
2). For the function f(x)=2x^3−6x+7, find all intervals where the function is increasing.

If a function is derivable over some interval, then it is increasing in this interval iff f'(x) > 0 in the interval. From here deduce where f increases and where it decreases.
Similarly, find all intervals where the function is decreasing.

I know you take the derivative, which is 6x^2-6. I believe then you set it equal to zero and then test points. However, I'm not getting the correct answer.

3). For the function f(x)=8x+5sin(x). The derivative is 5cosx+8. Find all intervals where the function is increasing.

Do as in (2)...you'll get a very nice and, perhaps, slighty unexpected answer Tonio
Similarly, find all intervals where the function is decreasing.

Please help!