\( tan \theta. tan \phi = 1\) হলে, \(sin\left(\cfrac{\theta+\phi}{3}\right)\) এর মান হবে (a) \(1\) (b) \(\cfrac{1}{\sqrt2}\) (c) \(\cfrac{\sqrt3}{2}\) (d) \(\cfrac{1}{2}\)
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Answer: D
\( tan \theta. tan \phi = 1\)
বা, \( tan \theta = \cfrac{1}{tan \phi}\)
বা, \( tan \theta = cot \phi\)
বা, \( tan \theta = tan(90^o- \phi)\)
বা, \( \theta = 90^o- \phi\)
বা, \( \theta+\phi = 90^o\)
বা, \(\cfrac{\theta+\phi}{3} = \cfrac{90^o}{3}\)
বা, \(sin\left(\cfrac{\theta+\phi}{3}\right) =sin 30^o=\cfrac{1}{2}\)

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