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7.2 definite integral — Maths STD 12 Science — Question

Gujarat BoardEnglish MediumSTD 12 ScienceMaths7.2 definite integral1 Mark
MCQ
If $\int \limits_{\frac{1}{3}}^3\left|\log _e x\right| d x=\frac{m}{n} \log _e\left(\frac{n^2}{e}\right)$, where $m$ and $n$ are coprime natural numbers, then $m ^2+ n ^2-5$ is equal to $............$.
  • ✓
    $20$
  • B
    $21$
  • C
    $22$
  • D
    $24$
✓

Answer

Correct option: A.
$20$
a
$\int \limits_{\frac{1}{3}}^3|\operatorname{nx}| dx =\int \limits_{\frac{1}{3}}^1(-\ell nx ) dx +\int_1^3(\ell nx ) dx$

$=-[ x \ell nx - x ]_{\ell / 3}^1+[ x \ell nx - x ]_1^3$

$=-\left[-1-\left(\frac{1}{3} \ell \ln \frac{1}{3}-\frac{1}{3}\right)\right]+[3 \ln 3-3-(-1)]$

$=\left[-\frac{2}{3}-\frac{1}{3} \ln \frac{1}{3}\right]+[3 \ln 3-2] ~\\ =-\frac{4}{3}+\frac{8}{3} \ln 3$

$=\frac{4}{3}(2 \ell n 3-1)$

$=\frac{4}{3}\left(\ln \frac{9}{ e }\right)$

$\therefore m =4, n =3$

$\text { Now }, m ^2+ n ^2-5=16+9-5=20$

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