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JEE Main 2021
Logarithms
Logarithm
Hard

Question

The number of integral solutions xx of log(x+72)(x72x3)20\log _{\left(x+\frac{7}{2}\right)}\left(\frac{x-7}{2 x-3}\right)^{2} \geq 0 is :

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Solution

log(x+72)(x72x3)20\log _{\left(x+\frac{7}{2}\right)}\left(\frac{x-7}{2 x-3}\right)^{2} \geq 0 Domain : x+72>0x>72x+721x52x72x30x7x32\begin{aligned} & x+\frac{7}{2}>0 \\\\ & x>\frac{-7}{2} \\\\ & x+\frac{7}{2} \neq 1 \\\\ & x \neq \frac{-5}{2} \\\\ & \frac{x-7}{2 x-3} \neq 0 \\\\ & x \neq 7 \\\\ & x \neq \frac{3}{2} \end{aligned}  Taking intersection : x(72,){52,32,7}\text { Taking intersection : } x \in\left(\frac{-7}{2}, \infty\right)-\left\{-\frac{5}{2}, \frac{3}{2}, 7\right\} Now logab0\log _{\mathrm{a}} \mathrm{b} \geq 0 if a>1\mathrm{a}>1 and b1\mathrm{b} \geq 1  Or a(0,1) and b(0,1)\begin{gathered} \text { Or } \\\\ a \in(0,1) \text { and } b \in(0,1) \end{gathered} Case I : x+72>1 and (x72x3)21x+\frac{7}{2}>1 \text { and }\left(\frac{x-7}{2 x-3}\right)^2 \geq 1 \therefore x>52x > -\frac{5}{2} and (2x3)2(x7)20(2x3+x7)(2x3x+7)0(3x10)(x+4)0\begin{aligned} & (2 x-3)^2-(x-7)^2 \leq 0 \\\\ & (2 x-3+x-7)(2 x-3-x+7) \leq 0 \\\\ & (3 x-10)(x+4) \leq 0 \end{aligned} x[4,103]x \in\left[-4, \frac{10}{3}\right]  Intersection : x(52,103]\text { Intersection : } \mathrm{x} \in\left(\frac{-5}{2}, \frac{10}{3}\right] Case II : x+72(0,1) and (x72x3)2(0,1)x+\frac{7}{2} \in(0,1) \text { and }\left(\frac{x-7}{2 x-3}\right)^2 \in(0,1) 0<x+72<172<x<52\begin{aligned} & \therefore 0 < x+\frac{7}{2}<1 \\\\ & -\frac{7}{2} < x < \frac{-5}{2} \end{aligned} and (x72x3)2<1(x7)2<(2x3)2\begin{aligned} & \left(\frac{x-7}{2 x-3}\right)^2<1 \\\\ & \Rightarrow (x-7)^2 < (2 x-3)^2 \end{aligned} \therefore x(,4)(103,)x \in(-\infty,-4) \cup\left(\frac{10}{3}, \infty\right) No common values of x\mathrm{x}. Hence intersection with feasible region. We get x(52,103]{32}x \in\left(\frac{-5}{2}, \frac{10}{3}\right]-\left\{\frac{3}{2}\right\} Integral value of xx are {2,1,0,1,2,3}\{-2,-1,0,1,2,3\} No. of integral values =6=6

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