Q.In a right-angled triangle, the two acute angles are \(\theta\) and \(\phi\). If \( \tan\theta = \cfrac{5}{12} \), then what is the value of \( \sin\phi \)? (a) \(\cfrac{12}{13}\) (b) \(\cfrac{5}{13}\) (c) \(\cfrac{1}{4}\) (d) \(\cfrac{10}{13}\)
Answer: A
Let \(\triangle ABC\) have \(\angle ACB = \theta\) and \(\angle BAC = \phi\) \(\therefore \tan \theta = \cfrac{AB}{BC} = \cfrac{5}{12}\) Let \(AB = 5x\) and \(BC = 12x\) \(\therefore AC = \sqrt{(5x)^2 + (12x)^2}\) \(= \sqrt{25x^2 + 144x^2}\) \(= \sqrt{169x^2} = 13x\) \(\therefore \sin \phi = \cfrac{BC}{AC} = \cfrac{12x}{13x} = \cfrac{12}{13}\)
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