Let the domain of the function \(\mathrm{f}(\mathrm{x})={\log }_{2}{\log }_{4}{\log }_{6}\left(3+4\mathrm{x}-{\mathrm{x}…
Let the domain of the function \(\mathrm{f}(\mathrm{x})={\log }_{2}{\log }_{4}{\log }_{6}\left(3+4\mathrm{x}-{\mathrm{x}}^{2}\right)\) be \((a,b)\). If \({\int }_{0}^{\mathrm{b}-\mathrm{a}}\left[{\mathrm{x}}^{2}\right]\mathrm{dx}\) \(=\mathrm{p}-\sqrt{\mathrm{q}}-\sqrt{\mathrm{r}},\mathrm{p},\mathrm{q},\mathrm{r}\in \mathrm{ℕ},\gcd (\mathrm{p},\mathrm{q},\mathrm{r})=1\), where \([\cdot ]\) is the greatest integer function, then \(\mathrm{p}+\mathrm{q}+\mathrm{r}\) is equal to
[JEE Main 2025, 3 Apr (Shift 1)]
\(10\)
\(f(x)={\log }_{2}{\log }_{4}{\log }_{6}(3+4x−{x}^{2})\)
\({\log }_{6}(3+4x−{x}^{2})>0\Rightarrow 3+4x−{x}^{2}>1\)
\(−{x}^{2}+4x+2>0\Rightarrow {x}^{2}−4x−2<0\)
Roots: \(x=2\pm \sqrt{6}\).
So, \(2−\sqrt{6} \({\log }_{4}({\log }_{6}(3+4x−{x}^{2}))>0\)\(\Rightarrow {\log }_{6}(3+4x−{x}^{2})>1\) \(3+4x−{x}^{2}>6\Rightarrow −{x}^{2}+4x−3>0\)\(\Rightarrow {x}^{2}−4x+3<0\) so \(1 \({\log }_{2}({\log }_{4}{\log }_{6}(⋅))\text{ requires }{\log }_{4}{\log }_{6}(⋅)>0,\) which is the same as step 2. So \(\text{domain}=(a,b)=(1,3)\mathrm{.}\) Thus \(b−a=2\) and we need \({\int }_{0}^{2}⌊{x}^{2}⌋\text{ }dx\mathrm{.}\) On \([0,2]\), \({x}^{2}\) runs from 0 to 4; break at where \({x}^{2}=k\): So \({\int }_{0}^{2}⌊{x}^{2}⌋dx={\int }_{0}^{1}0\text{ }dx+{\int }_{1}^{\sqrt{2}}1\text{ }dx\)\(+{\int }_{\sqrt{2}}^{\sqrt{3}}2\text{ }dx+{\int }_{\sqrt{3}}^{2}3\text{ }dx\) \(=(\sqrt{2}−1)+2(\sqrt{3}−\sqrt{2})+3(2−\sqrt{3})\mathrm{.}\) Simplify: \(=\sqrt{2}−1+2\sqrt{3}−2\sqrt{2}+6−3\sqrt{3}\)\(=5−\sqrt{2}−\sqrt{3}\mathrm{.}\) Thus it matches \(p−\sqrt{q}−\sqrt{r}\) with \(p+q+r=5+2+3=10.\)
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