\(\alpha,\ \beta\) and \(\gamma\) are angles in a triangle.
Show that \(\cos\gamma=-\cos(\alpha+\beta)\).[2]
Given that \(\sin\alpha=\dfrac5{13}\) and \(\cos\beta=-\dfrac35\), without solving for \(\alpha,\ \beta\) and \(\gamma\), find the value of \(\cos\gamma\).[5]
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