JCECE Engineering JCECE Engineering Solved Paper-2003

  • question_answer
    If\[f(x)=A\sin \left( \frac{\pi x}{2} \right)+B,\,\,\,f'\left( \frac{1}{2} \right)=\sqrt{2}\]and\[\int_{0}^{1}{f(x)}dx=\frac{2A}{\pi }\], then the constants \[A\] and \[B\] are respectively:

    A) \[\frac{\pi }{2}\]and\[\frac{\pi }{2}\]

    B) \[\frac{2}{\pi }\]and\[\frac{3}{\pi }\]

    C)  \[\frac{4}{\pi }\]and\[0\]

    D)  \[0\]and\[\frac{-4}{\pi }\]

    Correct Answer: C

    Solution :

    We have,\[f(x)=A\sin \left( \frac{\pi x}{2} \right)+B,\,\,f'\left( \frac{1}{2} \right)=\sqrt{2}\] and\[\int_{0}^{1}{f(x)}=\frac{2A}{\pi }\] \[\Rightarrow \]               \[\int_{0}^{1}{A\sin \left( \frac{\pi x}{2} \right)+B=\frac{2A}{\pi }}\] \[\Rightarrow \]               \[\left[ \frac{-2A\cos \left( \frac{\pi x}{2} \right)}{\pi }+Bx \right]=\frac{2A}{\pi }\] \[\Rightarrow \]               \[-\frac{2A}{\pi }\cos \left( \frac{\pi }{2} \right)+B+\frac{2A}{\pi }\cos {{0}^{o}}=\frac{2A}{\pi }\] \[\Rightarrow \]               \[B+\frac{2A}{\pi }=\frac{2A}{\pi }\] \[\Rightarrow \]               \[B=0\] Now,     \[f(x)=A\sin \left( \frac{\pi x}{2} \right)\] \[\Rightarrow \]               \[f'(x)=A\sin \left( \frac{\pi x}{2} \right)\] \[\Rightarrow \]               \[f'(x)=\frac{\pi A}{2}\cos \left( \frac{\pi x}{2} \right)\] \[\Rightarrow \]               \[f'\left( \frac{1}{2} \right)=\frac{\pi A}{2}\cos \left( \frac{\pi }{4} \right)\] \[\Rightarrow \]               \[A=\frac{\pi }{4}\] \[\therefore \]  \[A=\frac{4}{\pi },\,\,B=0\]


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