Inverse Circular Functions 2 Question 15

15. If f:[0,4π][0,π] be defined by f(x)=cos1(cosx). Then, the number of points x[0,4π] satisfying the equation f(x)=10x10, is

(2014 Adv.)

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Solution:

  1. PLAN

(i) Using definition of f(x)=cos1(x), we trace the curve f(x)=cos1(cosx)

(ii) The number of solutions of equations involving trigonometric and algebraic functions and involving both functions are found using graphs of the curves.

We know that, cos1(cosx)=x, if x[0,π] 2πx, if x[π,2π] 2π+x, if x[2π,3π] 4πx, if x[3π,4π]

From above graph, it is clear that y=10x10 and y=cos1(cosx) intersect at three distinct points, so number of solutions is 3 .



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