JEE Main & Advanced Mathematics Trigonometric Identities Question Bank Trigonometrical ratios of sum and difference of two and three angles

  • question_answer
    If \[\sin \theta =\frac{12}{13},(0<\theta <\frac{\pi }{2})\] and \[\cos \varphi =-\frac{3}{5},\left( \pi <\varphi <\frac{3\pi }{2} \right)\]. Then \[\sin (\theta +\varphi )\]will be [Orissa JEE 2004]

    A) \[\frac{-56}{61}\]

    B) \[\frac{-56}{65}\]

    C) \[\frac{1}{65}\]

    D) -56

    Correct Answer: B

    Solution :

    We have \[\sin \theta =\frac{12}{13}\] \[\cos \theta =\sqrt{1-{{\sin }^{2}}\theta }=\sqrt{1-{{\left( \frac{12}{13} \right)}^{2}}}=\frac{5}{13}\] and \[\cos \varphi =\frac{-3}{5},\sin \varphi =\sqrt{1-\frac{9}{25}}=\frac{-4}{5}\], \[\left[ \because \pi <\varphi <\frac{3\pi }{2} \right]\] Now, \[\sin (\theta +\varphi )=\sin \theta .\cos \varphi +\cos \theta .\sin \varphi \] \[=\left( \frac{12}{13} \right)\,\left( \frac{-3}{5} \right)+\left( \frac{5}{13} \right)\,\left( \frac{-4}{5} \right)=\frac{-36}{65}-\frac{20}{65}\]\[=\frac{-56}{65}\].


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