JEE Main & Advanced Mathematics Trigonometric Equations Question Bank Relation between sides and angles, Solutions of triangles

  • question_answer
    If \[{{a}^{2}},{{b}^{2}},{{c}^{2}}\]are in A. P. then which of the following are also in A.P. [ISM Dhandbad 1989]

    A) \[\sin A,\sin B,\sin C\]

    B) \[\tan A,\tan B,\tan C\]

    C) \[\cot A,\cot B,\cot C\]

    D) None of these

    Correct Answer: C

    Solution :

      \[{{\sin }^{2}}B-{{\sin }^{2}}A={{\sin }^{2}}C-{{\sin }^{2}}B\] \ \[\sin (B+A)\sin (B-A)=\sin (C+B)\sin (C-B)\] or \[\sin C(\sin B\cos A-\cos B\sin A)\]      \[=\sin A(\sin C\cos B-\cos C\sin B)\] Divide by \[\sin A\sin B\sin C\]         \[\therefore \,\,\,\cot A-\cot B=\cot B-\cot C\]. Hence the result.


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