## Friday, April 8, 2016

### Prove, with no knowledge of the decimal value of $\pi$ should be assumed or used that [MATH]1\lt \int_{3}^{5} \frac{1}{\sqrt{-x^2+8x-12}}\,dx \lt \frac{2\sqrt{3}}{3}[/MATH].

Prove, with no knowledge of the decimal value of $\pi$ should be assumed or used that [MATH]1\lt \int_{3}^{5} \frac{1}{\sqrt{-x^2+8x-12}}\,dx \lt \frac{2\sqrt{3}}{3}[/MATH].

The solution below is provided by MarkFL:

We are given to prove:

[MATH]1<\int_{3}^{5} \frac{1}{\sqrt{-x^2+8x-12}}\,dx<\frac{2}{\sqrt{3}}\tag{1}[/MATH]

If we move the integral 4 units to the left, and then use the even function rule, and divide through by 2, we obtain:

[MATH]\frac{1}{2}<\int_0^1\frac{1}{\sqrt{4-x^2}}\,dx<\frac{1}{\sqrt{3}}[/MATH]

If we define:

[MATH]f(x)=\frac{1}{\sqrt{4-x^2}}[/MATH]

then there results:

[MATH]f'(x)=\frac{x}{(4-x^2)^{\frac{3}{2}}}[/MATH]

Since the integrand is strictly increasing within the bounds, we know that the integral is greater than the left Riemann sum and less than the right sum. Using 1 partition, we may then write:

[MATH]f(0)<\int_0^1\frac{1}{\sqrt{4-x^2}}\,dx[/MATH]

Or:

[MATH]\frac{1}{2}<\int_0^1\frac{1}{\sqrt{4-x^2}}\,dx<\frac{1}{\sqrt{3}}[/MATH]

Shown as desired.