# Integral -- analytical way to prove this integral is non-negative?

1. Aug 1, 2014

### Mark_M

Is there any analytical way to prove that the integral $$\int_{2.04}^\infty \frac{\sin x}{x^2}dx$$ is nonegative?

I tryed to use geometrical approach, i.e. the graph of the integrand look like:

The magnitude became smaller and smaller, since $$\sin x$$ is multiplied by the decreasing function $1/x^2$, so the third area, which is positive, is bigger then the forth one, which is negative, and so on. BUT I dont know what to do with the first two areas(

OR integration by parts gave me
$$\int_{2.04}^\infty \frac{\sin x}{x^2}dx=\frac{\sin(2.04)}{2.04}-Ci(2.04)$$, where $$Ci(x)$$ is the cosine integral function.

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Last edited: Aug 1, 2014
2. Aug 2, 2014

### pasmith

So all you need to know is whether or not $$\int_{2.04}^{2\pi} \frac{\sin x}{x^2}\,dx \geq 0.$$ Evaluate it numerically.

If that doesn't satisfy your "analytic way" criterion, then you can instead show that there exists a lower Darboux sum which is strictly positive. It then follows that the integral is strictly positive.