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Limits — Maths STD 11 Science — Question

CBSE BoardEnglish MediumSTD 11 ScienceMathsLimits5 Marks
Question
Evaluate the following limit:
$\lim\limits_{\text{x}\rightarrow{\frac{\pi}{2}}}\frac{\big(\frac\pi2-\text{x}\big)\sin\text{x}-2\cos\text{x}}{\big(\frac\pi2-\text{x}\big)+\cot\text{x}}$
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Answer

$\lim\limits_{\text{x}\rightarrow{\frac{\pi}{2}}}\frac{\big(\frac\pi2-\text{x}\big)\sin\text{x}-2\cos\text{x}}{\big(\frac\pi2-\text{x}\big)+\cot\text{x}}$
If $\text{x}\rightarrow\frac{\pi}{2},$ then $\frac\pi2-\text{x}\rightarrow0$ let $\frac\pi2-\text{x}=\text{y}$
$=\lim\limits_{\text{y}\rightarrow{0}}\frac{\Big(\text{y}\sin\big(\frac{\pi}{2}-\text{y}\big)-2\cos\big(\frac{\pi}{2}-\text{y}\big)\Big)}{\text{y}+\cot\big(\frac\pi2-\text{y}\big)}$
$=\lim\limits_{\text{y}\rightarrow{0}}\Big(\frac{\text{y}\cos\text{y}-2\sin\text{y}}{1+\tan\text{y}}\Big)$
$=\lim\limits_{\text{y}\rightarrow{0}}\Bigg(\frac{\cos\text{y}-2\frac{\sin\text{y}}{\text{y}}}{1+\frac{\tan\text{y}}{\text{y}}}\Bigg)$
$=\frac{\lim\limits_{\text{y}\rightarrow{0}}\cos\text{y}-2\lim\limits_{\text{y}\rightarrow{0}}\frac{\sin\text{y}}{\text{y}}}{1+\lim\limits_{\text{y}\rightarrow{0}}\frac{\tan\text{y}}{\text{y}}}$ $\Big[\because\lim\limits_{\text{y}\rightarrow0}\frac{\sin\text{y}}{\text{y}}=1,\lim\limits_{\text{y}\rightarrow0}\frac{\tan\text{y}}{\text{y}}=1\Big]$
$=\frac{1-2}{1+1}=\frac{-1}{2}$
$=-\frac12$

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