10th Class Mathematics Introduction to Trigonometry Question Bank MCQs - Introduction to Trigonometry

  • question_answer
    If \[x{{\sin }^{3}}\theta +y{{\cos }^{3}}\theta =\sin \theta \cos \theta \]and \[x\sin \theta =y\cos \theta ,\]than \[{{x}^{2}}+{{y}^{2}}\] is equal to:

    A) \[0\]

    B) \[1/2\]

    C) \[1\]

    D) \[3/2\]

    Correct Answer: C

    Solution :

    [c] We have,  \[x{{\sin }^{3}}\theta +y{{\cos }^{3}}\theta =\sin \theta \cos \theta \]
    \[(x\sin \theta ){{\sin }^{2}}\theta +(y\cos \theta ){{\cos }^{2}}\theta =\sin \theta \cos \theta \]
    \[x\sin \theta ({{\sin }^{2}}\theta )+(x\sin \theta ){{\cos }^{2}}\theta =\sin \theta \cos \theta \]
    \[x\sin \theta ({{\sin }^{2}}\theta +{{\cos }^{2}}\theta )=\sin \theta \cos \theta \]
    \[x\sin \theta =\sin \theta \cos \theta \,\,\,\,\,\,\Rightarrow \,\,\,\,\,\,\,\,x=\cos \theta \]
    Now,\[x\sin \theta =y\cos \theta \]
    \[\cos \theta \sin \theta =y\cos \theta \]
    \[y=\sin \theta \]
    Hence, \[{{x}^{2}}+{{y}^{2}}={{\cos }^{2}}\theta +{{\sin }^{2}}\theta =1\]


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