# Homework Help: Taylor innequality prove

1. Feb 28, 2009

### transgalactic

prove this inequality for x>0
$$x-\frac{x^3}{6}+\frac{x^5}{120}>\sin x$$

this is a tailor series for sin x
$$sinx=x-\frac{x^3}{6}+\frac{x^5}{120}+R_5$$
for this innequality to be correct the remainder must be negative
but i cant prove it because there are values for c when the -sin c expression will be possitive

$$R_5=\frac{-sin(c)}{6!}x^6$$

??

i got another approach
$$f(x)=x-\frac{x^3}{6}+\frac{x^5}{120}-sin$$
$$f'(x)=1-\frac{x^2}{2}x+\frac{x^4}{24}-\cos x$$
$$f''(x)=-x+\frac{x^3}{6}+\sin x$$
$$f^{(3)}(x)=-1+\frac{x^2}{2}+\cos x$$
$$f^{(4)}(x)=x-\sin x$$
in this point how to know that the 4th derivative is always positive for x>0

??

2. Feb 28, 2009

### Staff: Mentor

Yes, this is a Taylor's series, but more precisely it is a Maclaurin series since it is in powers of x or x - 0.
I seem to remember that you asked this same question before. How did it turn out that time?
The remainder formula gives bounds on c. What are they for your problem? Your problem also states what values of x to consider. Understanding the bounds on c and the possible values for x will help you with evaluating -sin(c).

3. Feb 28, 2009

### transgalactic

in this point how to know that the 4th derivative is always positive for x>0

??

4. Feb 28, 2009

### Staff: Mentor

Did you look at what I wrote in my previous post?

5. Mar 1, 2009

### transgalactic

you say that i should bound c
so the -sinc will give a ngative value

but i cent do it
i have x>0

and in this area it can be both possitive and negative .

i prefer to shoe that the 4th derivative is always positive for x>0

how to show that?

6. Mar 1, 2009

### Staff: Mentor

I don't understand what you're doing, but here's some help at what you're trying to do.
g(x) = x - sin(x) is 0 at x = 0, and g'(x) > 0 except at x = k*pi. That should tell you something about the sign of x - sin(x).

7. Mar 2, 2009

### transgalactic

its not proving that the 4th derivative is positive

8. Mar 2, 2009

### Gib Z

Really, at the level of answering Taylor Series questions I don't think anyone would think twice about you simply stating that the inequality x > sin x, x>0 holds.

Once you agree on that, one could integrate both sides of that inequality between 0 and some number, with gives us an inequality for cos in terms of t. Then integrate that one from 0 to x, you'll see where this is getting at.

9. Mar 2, 2009

### transgalactic

what do you man that level
i cant just say that x>sinx
x>0
i need to base this fact on some math or whatever
??