JEE Main & Advanced Mathematics Trigonometric Equations Question Bank Solution of trigonometrical equations

  • question_answer
    If \[\sin \theta +\cos \theta =1\] then the general value of \[\theta \]is [MNR 1987; IIT 1981; Karnataka CET 2000, 03; DCE 2000]

    A) \[2n\pi \]

    B) \[n\pi +{{(-1)}^{n}}\frac{\pi }{4}-\frac{\pi }{4}\]

    C) \[2n\pi +\frac{\pi }{2}\]

    D) None of these

    Correct Answer: B

    Solution :

    \[\sin \theta +\cos \theta =1\Rightarrow \frac{1}{\sqrt{2}}\sin \theta +\frac{1}{\sqrt{2}}\cos \theta =\frac{1}{\sqrt{2}}\] Dividing by\[\sqrt{{{1}^{2}}+{{1}^{2}}}=\sqrt{2}\], we get   \[\sin \left( \theta +\frac{\pi }{4} \right)=\frac{1}{\sqrt{2}}=\sin \frac{\pi }{4}\] \[\Rightarrow \] \[\theta +\frac{\pi }{4}=n\pi +{{(-1)}^{n}}\frac{\pi }{4}\Rightarrow \theta =n\pi +{{(-1)}^{n}}\frac{\pi }{4}-\frac{\pi }{4}\].


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