The harmonic conjugate of $(2,3,4)$ with respect to the points $(3,-2,2)$ and $(6,-17,-4)$ is

The harmonic conjugate of $(2,3,4)$ with respect to the points $(3,-2,2)$ and $(6,-17,-4)$ is
  1. $(11,-16,2)$
  2. $\left(\frac{1}{2}, \frac{1}{3}, \frac{1}{4}\right)$
  3. $(0,0,0)$
  4. $\left(\frac{18}{5}, \frac{-5}{1}, \frac{4}{5}\right)$

Solution

Let $A=(3,-2,2)$ $ \begin{aligned} & B=(6,-17,-4) \\ & P=(2,3,4) \end{aligned} $ Let $P$ divides $A B$ in the ratio of $m: n$. $ \begin{gathered} P=\left[\frac{m x_2+n x_1}{m+n}, \frac{m y_2+n y_1}{m+n}, \frac{m z_2+n z_1}{m+n}\right] \\ (2,3,4)=\left[\frac{m(6)+n(3)}{m+n}, \frac{m(-17)+n(-2)}{m+n},\right. \\ (2,3,4)=\left[\frac{6 m(-4)+n(2)}{m+n}\right] \\ \frac{6 m+3 n}{m+n}=2 \\ 6 m+3 n=2 m+2 n \\ 4 m=-n \\ \frac{m}{n}=-\frac{1}{4} \\ m: n=-1: 4 \end{gathered} $ We know that, If point $P$ divides a line segment in the ratio of $m: n$, then its harmonic conjugate will divide same segment in the ratio of $-m: n$ $\therefore$ Required ratio $=-m: n=1: 4$ $\therefore$ Required harmonic conjugate $ \begin{aligned} & =\left(\frac{m x_2+n x_1}{m+n}, \frac{m y_2+n y_1}{m+n}, \frac{m z_2+n z_1}{m+n}\right) \\ & =\left(\frac{1(6)+4(3)}{1+4}, \frac{1(-17)+4(-2)}{1+4}, \frac{1(-4)+4(2)}{1+4}\right) \\ & =\left(\frac{18}{5}, \frac{-25}{5}, \frac{4}{5}\right)=\left(\frac{18}{5},-5, \frac{4}{5}\right) \end{aligned} $ Hence option (4) is correct

Asked in: AP EAMCET 2020 (22 Sep Shift 2)

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