I have a bit of trouble with that because my proof attempt is breaking. Down.

Evidence:

Just show that $ f (x) = sin (x) – x + frac {x ^ 3} {3!}> 0 $ sure $ I = (0, sqrt {20}) $. This is true if $ f (x) $ strictly increases on the interval and $ f (0) geq 0 $. We can also apply this property to the first and second derivatives. We note $ f ^ {(3)} (x) = – cos (x) + 1 $. However, I can only show that $ f ^ {(3)} (x) geq 0 $ sure $ I $ from home $ x = frac {3 pi} {2} $ $ f ^ {(3)} (x) = 0 $