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

  • question_answer
    If \[\cos P=\frac{1}{7}\] and \[\cos Q=\frac{13}{14},\] where P and Q both are acute angles. Then the value of \[P-Q\] is [Karnataka CET 2002]

    A) \[{{30}^{o}}\]

    B) \[{{60}^{o}}\]

    C) \[{{45}^{o}}\]

    D) \[{{75}^{o}}\]

    Correct Answer: B

    Solution :

    Given, \[\cos P=\frac{1}{7},\cos Q=\frac{13}{14}\] \\[\cos (P-Q)=\cos P\cos Q+\sin P\sin Q\] \[=\frac{1}{7}.\frac{13}{14}+\frac{\sqrt{48}}{7}.\frac{\sqrt{27}}{14}=\frac{13+36}{98}=\frac{1}{2}=\cos {{60}^{o}}\] Þ \[P-Q={{60}^{o}}\].


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