If $a, b$ and $c$ are real numbers such that $a^2+b^2+c^2-a b-b c-a c \leq 0$, then $$…

If $a, b$ and $c$ are real numbers such that $a^2+b^2+c^2-a b-b c-a c \leq 0$, then $$ \left|\begin{array}{ccc} (a-b+1)^5 & b^7-c^7 & c^9-a^9 \\ a^{11}-b^{11} & (b-c+2)^3 & c^{13}-a^{13} \\ a^{15}-b^{15} & b^{17}-c^{17} & (c-a+3)^1 \end{array}\right|= $$
  1. $2 a b c$
  2. 0
  3. $24 a b c$
  4. 24

Solution

Given, $a^2+b^2+c^2-a b-b c-c a \leq 0$ $ \therefore \frac{1}{2}\left[(a-b)^2+(b-c)^2+(c-a)^2\right] \leq 0 $ It is possible only $a=b=c$ $ \begin{aligned} & \text { Now, }\left|\begin{array}{ccc} (a-b+1)^5 & b^7-c^7 & c^9-a^9 \\ a^{11}-b^{11} & (b-c+2)^3 & c^{13}-a^{13} \\ a^{15}-b^{15} & b^{17}-c^{17} & (c-a+3) 1 \end{array}\right| \\ & \Rightarrow\left|\begin{array}{lll} 1 & 0 & 0 \\ 0 & 8 & 0 \\ 0 & 0 & 3 \end{array}\right|=24 \end{aligned} $

Asked in: AP EAMCET 2021 (25 Aug Shift 1)

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