JCECE Engineering JCECE Engineering Solved Paper-2003

  • question_answer
    \[\int{\frac{{{\sin }^{8}}x-{{\cos }^{8}}x}{1-2{{\sin }^{2}}x{{\cos }^{2}}x}dx}\]is equal to:

    A) \[\sin 2x+c\]

    B) \[-\frac{1}{2}\sin 2x+c\]

    C)   \[\frac{1}{2}\sin 2x+c\]

    D)  \[-\sin 2x+c\]

    Correct Answer: B

    Solution :

    Let\[\int{=\frac{{{\sin }^{8}}x-{{\cos }^{8}}x}{1-2{{\sin }^{2}}x{{\cos }^{2}}x}dx}\]                 \[=\int{\frac{({{\sin }^{4}}x-{{\cos }^{4}}x)({{\sin }^{4}}x+{{\cos }^{4}}x)}{({{\sin }^{2}}x+{{\cos }^{2}}x)-2{{\sin }^{2}}x{{\cos }^{2}}x}}dx\]                 \[=\int{\frac{({{\sin }^{4}}x-{{\cos }^{4}}x)({{\sin }^{4}}x+{{\cos }^{4}}x)}{({{\sin }^{4}}x+{{\cos }^{4}}x)}dx}\]                 \[=\int{({{\sin }^{4}}x-{{\cos }^{4}}x)dx}\]                 \[=\int{({{\sin }^{2}}x-{{\cos }^{2}}x)({{\sin }^{2}}x+{{\cos }^{2}}x)dx}\]                 \[=-\int{({{\cos }^{2}}x-{{\sin }^{2}}x)}dx\]                 \[=-\int{\cos 2\,\,x\,\,dx}\]                 \[=-\frac{\sin 2x}{2}+c\]


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