In $\triangle P Q R, \angle R=\frac{\pi}{4}, \tan \left(\frac{P}{3}\right), \tan \left(\frac{Q}{3}\right)$…

In $\triangle P Q R, \angle R=\frac{\pi}{4}, \tan \left(\frac{P}{3}\right), \tan \left(\frac{Q}{3}\right)$ are the roots of the equation $a x^2+b x+c=0$, then
  1. $a+b=c$
  2. $b+c=0$
  3. $a+c=0$
  4. $b=c$

Solution

Given, $\quad R=\frac{\pi}{4}$ Also, $P+Q+R=\pi$ $\begin{array}{ll}\Rightarrow & P+Q+\frac{\pi}{4}=\pi \\ \Rightarrow & P+Q=\frac{3 \pi}{4} \\ \Rightarrow & \frac{P}{3}+\frac{Q}{3}=\frac{\pi}{4} \\ \Rightarrow \quad & \tan \left(\frac{P}{3}+\frac{Q}{3}\right)=\tan \left(\frac{\pi}{4}\right) \\ \Rightarrow \quad & \frac{\tan \frac{P}{3}+\tan \frac{Q}{3}}{1-\tan \frac{P}{3} \tan \frac{Q}{3}}=1\end{array}$ Since, $\tan \frac{P}{3}$ and $\tan \frac{Q}{3}$ are the roots of the equation $a x^2+b x+c=0$. $\therefore \quad \tan \frac{P}{3}+\tan \frac{Q}{3}=-\frac{b}{a}$ and $\tan \frac{P}{3} \cdot \tan \frac{Q}{3}=\frac{c}{a}$ $\therefore$ From Eq. (i), $\frac{-\frac{b}{a}}{1-\frac{c}{a}}=1$ $\begin{array}{ll}\Rightarrow & \frac{-b}{a-c}=1 \\ \Rightarrow & a+b=c\end{array}$

Asked in: AP EAMCET 2012

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