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NDA 2015
Exam
NDA
Year
2015
Subject
Mathematics
Chapter
Determinants

NDA 2015 · Mathematics · Determinants

If the value of the determinant | array lll a & 1 & 1 1 & ~b & 1 1...

If the value of the determinant \left|a111 b111c\begin{array}{lll}\mathrm{a} & 1 & 1 \\ 1 & \mathrm{~b} & 1 \\ 1 & 1 & \mathrm{c}\end{array}\right| is positive, where a≠b≠ca \neq b \neq c, then the value of abca b c

Options

  1. Acannot be less than 1
  2. Bis greater than -8
  3. Cis less than -8
  4. Dmust be greater than 8

Correct answer

B. is greater than -8

Step-by-step solution

∣a111b111c∣>0⇒a(bc−1)−1(c−1)+1(1−b)>0⇒abc−a−c+1+1−b>0⇒abc+2−(a+b+c)>0\begin{aligned} & \left|\begin{array}{lll}\mathrm{a} & 1 & 1 \\ 1 & \mathrm{ b} & 1 \\ 1 & 1 & \mathrm{c}\end{array}\right| > 0 \\ & \Rightarrow \mathrm{a}(\mathrm{bc}-1)-1(\mathrm{c}-1)+1(1-\mathrm{b}) > 0 \\ & \Rightarrow \mathrm{abc}-\mathrm{a}-\mathrm{c}+1+1-\mathrm{b} > 0 \\ & \Rightarrow \mathrm{abc}+2-(\mathrm{a}+\mathrm{b}+\mathrm{c}) > 0\end{aligned}\Rightarrow \mathrm{abc} > (\mathrm{a}+\mathrm{b}+\mathrm{c})-2Let;Let;\mathrm{a}=-1 ; \mathrm{b}=0 \ \text{\text{ and } } \mathrm{c}=1Then;Then;0 > -2[whichiscorrect]Hence,[which is correct] Hence,a b c=0\therefore$ After considering all the option; (2) is correct option.
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