JEE Advanced · Mathematics · 17. Properties of Triangles
Internal bisector of \(\angle A\) of \(\triangle A B C\) meets side \(B C\) at \(D\). A line drawn through \(D\) perpendicular to \(A D\) intersects the side \(A C\) at \(E\) and side \(A B\) at \(F\). If \(a, b\) and \(c\) represent sides of \(\triangle A B C\), then
- A
\(A E\) is \(\mathrm{HM}\) of \(b\) and \(c\)
- B
\(A D=\frac{2 b c}{b+c} \cos \frac{A}{2}\)
- C
\(E F=\frac{4 b c}{b+c} \sin \frac{A}{2}\)
- D
the \(\triangle A E F\) is isosceles
Answer & Solution
Correct Answer
(A)
\(A E\) is \(\mathrm{HM}\) of \(b\) and \(c\)
Step-by-step Solution
Detailed explanation
We have, \(\triangle A B C=\triangle A B D+\triangle A C D\)
\[
\begin{aligned}
& \Rightarrow \frac{1}{2} b c \sin A=\frac{1}{2} c A D \sin \frac{A}{2}+\frac{1}{2} b A D \sin \frac{A}{2} \\
& \Rightarrow \quad A D=\frac{2 b c}{b+c} \cos \frac{A}{2}
\end{aligned}
\]
Again, \(\quad A E=A D \sec \frac{A}{2}=\frac{2 b c}{b+c}\)
\(\therefore A E\) is \(\mathrm{HM}\) of \(b\) and \(c\).
\[
\begin{aligned}
E F & =E D+D F=2 D E=2 A D \tan \frac{A}{2} \\
& =2 \frac{2 b c}{b+c} \cos \frac{A}{2} \tan \frac{A}{2}=\frac{4 b c}{b+c} \sin \frac{A}{2}
\end{aligned}
\]
As \(A D \perp E F\) and \(D E=D F\) and \(A D\) is bisector.
\(\Rightarrow \triangle A E F\) is isosceles.

\[
\begin{aligned}
& \Rightarrow \frac{1}{2} b c \sin A=\frac{1}{2} c A D \sin \frac{A}{2}+\frac{1}{2} b A D \sin \frac{A}{2} \\
& \Rightarrow \quad A D=\frac{2 b c}{b+c} \cos \frac{A}{2}
\end{aligned}
\]
Again, \(\quad A E=A D \sec \frac{A}{2}=\frac{2 b c}{b+c}\)
\(\therefore A E\) is \(\mathrm{HM}\) of \(b\) and \(c\).
\[
\begin{aligned}
E F & =E D+D F=2 D E=2 A D \tan \frac{A}{2} \\
& =2 \frac{2 b c}{b+c} \cos \frac{A}{2} \tan \frac{A}{2}=\frac{4 b c}{b+c} \sin \frac{A}{2}
\end{aligned}
\]
As \(A D \perp E F\) and \(D E=D F\) and \(A D\) is bisector.
\(\Rightarrow \triangle A E F\) is isosceles.

See the Complete Solution
Get step-by-step explanations for this and 2.5 Lakh+ more JEE, NEET & CET questions.
- Unlock all solutions
- Practice the full chapter
- Track accuracy across PYQs
4.8 rated on Google Play · 14,000+ reviews
More questions from Mathematics
- Let \(X=\left({ }^{10} C_1\right)^2+2\left({ }^{10} C_2\right)^2+3\left({ }^{10} C_3\right)^2+\ldots+\) \(10\left({ }^{10} C_{10}\right)^2\), where \({ }^{10} C_r, r \in\{1,2, \ldots, 10\}\) denote binomial coefficients. Then, the value of \(\frac{1}{1430} X\) is _____________.JEE Advanced 2018 Hard
- Paragraph:
Let \(p\) be an odd prime number and \(T_p\) be the following set of \(2 \times 2\) matrices
\[
T_p=\left\{A=\left[\begin{array}{ll}
a & b \\
c & a
\end{array}\right] ; a, b, c \in\{0,1,2, \ldots, p-1\}\right\}
\]Question:
The number of \(A\) in \(T_p\) such that \(A\) is either symmetric or skew-symmetric or both, and det \((A)\) is divisible by \(p\) isJEE Advanced 2010 Hard - The area of the region isJEE Advanced 2019 Easy
- In a high school, a committee has to be formed from a group of boys
and girls .
(i) Let be the total number of ways in which the committee can be formed such that the committee has members, having exactly boys and girls.
(ii) Let be the total number of ways in which the committee can be formed such that the committee has at least members, and having an equal number of boys and girls.
(iii) Let be the total number of ways in which the committee can be formed such that the committee has members, at least of them being girls.
(iv) Let be the total number of ways in which the committee can be formed such that the committee has members, having at least girls and such that both and are NOT in the committee together.
The correct option is:LIST-I LIST-II A. The value of is P. B. The value of is Q. C. The value of is R. D. The value of is S. T. U. JEE Advanced 2018 Medium - Paragraph:
Let \(V_r\) denotes the sum of the first \(r\) terms of an arithmetic progression \((A P)\) whose first term is \(r\) and the common difference is \((2 r-1)\). Let \(T_r=V_{r+1}-V_r-2\) and \(Q_r=T_{r+1}-T_r\) for \(r=1,2, \ldots\)
Question:
The sum \(V_1+V_2+\ldots+V_n\) isJEE Advanced 2007 Medium - Three numbers are chosen at random, one after another with replacement, from the set \(S=\{1,2,3, \ldots, 100\}\). Let \(p_{1}\) be the probability that the maximum of chosen numbers is at least 81 and \(p_{2}\) be the probability that the minimum of chosen numbers is at most 40 .
The value of isJEE Advanced 2021 Medium
More PYQs from JEE Advanced
- Consider the following molecules : , and . Count the number of atoms existing in their zero oxidation state in each molecule. Their sum isJEE Advanced 2023 Easy
- Match each of the compounds is Column I with its characteristic reaction(s) is Column II.
JEE Advanced 2009 Hard - On decreasing the from to , the solubility of a sparingly soluble salt of a weak acid increased from to . The of isJEE Advanced 2023 Easy
- The major product of the following reaction is
JEE Advanced 2011 Hard - Assume that the nuclear binding energy per nucleon \((B / A)\) versus mass number \((A)\) is as shown in the figure. Use this plot to choose the correct choice (s) given below.
JEE Advanced 2008 Medium - Among the following, the number of elements showing only one non-zero oxidation state is O, Cl, F, N, P, Sn, Tl, \(\mathrm{Na}, \mathrm{Ti}\)JEE Advanced 2010 Easy