Equating the second derivative to zero, we find:
Now, use the double-angle identity for sine, then factor. What do you find?
so here's the function
f(x) = 2sinx + sin2x and in order to find the concavity i need to find the second derivative which is this.... f ''(x)= -2sinx - 4sin2x.... now how do i find the critical numbers to determine the concavity of the function? how to i set x equal to zero to find the critical numbers.... please help!!