If the function is twice differentiable at , then the graph of is concave upward at
if and concave downward if
Now we know that:
To find the concavity we take the second derivative of the function, equal it to zero and find the values of . Those values of are the points(inflection points) where concavity changes.
So the function changes concavity times where within the interval .
You can see the plot of the function here:
plot x^2 + 5cos(x), x = 0 to 720 degree - Wolfram|Alpha