Proving Dn Non-Abelian for n >= 3

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My goal is to prove that if n >= 3 then Dn, which is the dihedral group of size n is non-abelian.

I am stuck, but my attempt at a solution was to show that some the symmetries in Dn are not commutative under composition, but I do not know how to prove that for n cases. My other attempt was to find a isomorphism to Dn and show that its isomorphism is not abelian, but I have no clue where to start from there.

I would appreciate either an answer, or something to help me get going. Please be precise and I thank you so much in advance.
 
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In general, a rotation doesn't commute with a reflection.
 

Thread 'Prove that the integral is equal to ##\pi^2/8##'

Prove $$\int\limits_0^{\sqrt2/4}\frac{1}{\sqrt{x-x^2}}\arcsin\sqrt{\frac{(x-1)\left(x-1+x\sqrt{9-16x}\right)}{1-2x}} \, \mathrm dx = \frac{\pi^2}{8}.$$ Let $$I = \int\limits_0^{\sqrt 2 / 4}\frac{1}{\sqrt{x-x^2}}\arcsin\sqrt{\frac{(x-1)\left(x-1+x\sqrt{9-16x}\right)}{1-2x}} \, \mathrm dx. \tag{1}$$ The representation integral of ##\arcsin## is $$\arcsin u = \int\limits_{0}^{1} \frac{\mathrm dt}{\sqrt{1-t^2}}, \qquad 0 \leqslant u \leqslant 1.$$ Plugging identity above into ##(1)## with ##u...
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