Trig question, help appreciated! :)

$An\ acute\ angle\ is\ an\ angle\ whose\ sides\ are\ within\ >\ 0^{\circ}\ and\ <\ 90^{\circ}\ \\ or\ whose\ circular\ arc\ are\ within\ >\ 0\ and\ <\ \dfrac{\pi}{2}.\\ \cos \theta = \frac{3 \sqrt{5}}{7}\\ sin \theta=\sqrt{1-cos^2}=\sqrt{1-\dfrac{9\cdot 5}{49}}=\dfrac{1}{7}\sqrt{49-45}=\dfrac{2}{7}\\ \color{blue}sin \theta=\dfrac{2}{7}$

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