JEE Main & Advanced JEE Main Paper (Held on 7 May 2012)

  • question_answer
    Let fix)be an indefinite integral of \[{{\cos }^{3}}x.\] Statement 1:f(x) is a periodic function of period\[\pi .\] Statement 2: \[{{\cos }^{3}}x\] is a periodic function.   JEE Main Online Paper (Held On 07 May 2012)

    A) Statement 1 is true, Statement 2 is false.

    B)                        Both the Statements are true, but Statement2 is not the correct explanation of Statement1.

    C)                        Both the Statements are true, and Statement2 is correct explanation of Statement 1.

    D)                        Statement 1 is false, Statement 2 is true.

    Correct Answer: D

    Solution :

                    Statement - 2: \[{{\cos }^{3}}x\] is a periodic function. It is a true statement. Statement-1 Given \[f(x)=\int_{{}}^{{}}{{{\cos }^{3}}x\,dx}\] \[=\int_{{}}^{{}}{\left( \frac{\cos 3x}{4}+\frac{3\cos x}{4} \right)dx}\] \[=\frac{1}{4}\frac{\sin 3x}{3}+\frac{3}{4}\sin x\] \[=\frac{1}{12}\sin 3x+\frac{3}{4}\sin x\] Now, period of \[\frac{1}{12}\sin 3x=\frac{2\pi }{3}\] Period of\[\frac{3}{4}\sin x=2\pi \] Hence period of \[f(x)=\frac{L.C.M.\left( 2\pi ,2\pi  \right)}{HCF\,\text{of}\,\left( 1,3 \right)}\] \[=\frac{2\pi }{1}=2\pi \] Thus,y(x) is aperiodic function of period \[2\pi .\] Hence, Statement -1 is false.


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