MCQ
The minimum value of ${\left( {\frac{3}{a} - 1} \right)^2} + {\left( {\frac{a}{b} - 1} \right)^2} + {\left( {\frac{b}{c} - 1} \right)^2} + {\left( {3c - 1} \right)^2}$ where $0\, < a,\,b,\,c\, \leqslant \,9$ ,is $p - q\sqrt r $ ; $p,q,r \in I$ and $q$ , $r$ are coprimes, then $(p + q + r)$ is equal to
  • A
    $16$
  • B
    $24$
  • $27$
  • D
    $30$

Answer

Correct option: C.
$27$
c
$\mathrm{z}=\left(\frac{3}{\mathrm{a}}-1\right)^{2}+\left(\frac{\mathrm{a}}{\mathrm{b}}-1\right)^{2}+\left(\frac{\mathrm{b}}{\mathrm{c}}-1\right)^{2}+(3 \mathrm{c}-1)^{2}$

$=\left(\frac{9}{a^{2}}+\frac{a^{2}}{b^{2}}+\frac{b^{2}}{c^{2}}+9 c^{2}\right)+4-\left(\frac{6}{a}+\frac{2 a}{b}+\frac{2 b}{c}+6 c\right)$

Now $\frac{{\frac{9}{{{a^2}}} + \frac{{{a^2}}}{{{b^2}}} + \frac{{{b^2}}}{{{c^2}}} + 9{c^2}}}{4} \ge \sqrt [4] {\frac{9}{{{a^2}}} \cdot \frac{{{a^2}}}{{{b^2}}} \cdot \frac{{{b^2}}}{{{c^2}}} \cdot 9{c^2}}  \ge 3$

 and $\frac{\frac{6}{a}+\frac{2 a}{b}+\frac{2 b}{c}+6 c}{4} \geq \sqrt[4]{\frac{6}{a} \cdot \frac{2 a}{b} \cdot \frac{2 b}{c} \cdot 6 c} \geq 2 \sqrt{3}$

$\therefore z \geq 12+4-8 \sqrt{3}=16-8 \sqrt{3}$

$\Rightarrow p+q+r=27$

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