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      Question

      Simplify the following trigonometric expression: 

      15 cos 25° cosec 65° − 5 cot 80° cot 10°
      B 10 Correct Answer Incorrect Answer
      C 5 Correct Answer Incorrect Answer
      D 20 Correct Answer Incorrect Answer

      Solution

      15 cos 25° cosec 65° − 5 cot 80° cot 10°

      = 15 cos 25° cosec (90° − 25°) − 5 cot 80° cot (90° − 80°)

      Using, cosec (90° − A) = sec A and cot (90° − A) = tan A

      = 15 cos 25° sec 25° − 5 cot 80° tan 80°

      Since, sec A is reciprocal of cos A and cot A is reciprocal of tan A.

      = 15 − 5 = 10

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