Your second derivative's last term should be -4sin2x
set y'' = 0 for points of inflection
-2cosx - 4sin2x = 0
cosx + 2sin2x = 0
cosx + 4sinxcosx = 0
cosx(1 + 4cosx) = 0
cosx = 0 or cosx = -1/4
You seem to know what you are doing, and can probably take it from here.
Concavity in short:
If for some f(x), the second derivative exists,
the curve is concave upwards if f''(x) > 0
the curve is concave downwards if f''(x) < 0
if f''(x) = 0 it is neither and you have a point of inflection as seen above.
from cosx + 4sinxcosx = 0
it should have been
cosx(1 + 4sinx) = 0
cosx = 0 or sinx = -1/4
I got-4sin2x for the second term, I made a typo.
How do you translate sinx=-1/4 to decimal numbers?
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