If in a triangle ABC, cos A + 2cos B + cos C = 2, then a,b,c are in

see full answer

Your Exam Success, Personally Taken Care Of

1:1 expert mentors customize learning to your strength and weaknesses – so you score higher in school , IIT JEE and NEET entrance exams.

An Intiative by Sri Chaitanya

A.P

G.P

H.P

None

answer is A.

(Unlock A.I Detailed Solution for FREE)

Best Courses for You

JEE

NEET

Foundation JEE

Foundation NEET

CBSE

Detailed Solution

$\cos A + 2 \cos B + \cos C = 2$
$\cos A + \cos C = 2 (1 - \cos B)$
$\begin{array}{l} o r 2 \cos \frac{A + C}{2} \cos \frac{A - C}{2} = 4 \sin^{2} \frac{B}{2} \\ o r \cos \frac{A - C}{2} = 2 \sin \frac{B}{2} = 2 \cos \frac{A + C}{2} \\ o r \cos \frac{A}{2} \cos \frac{C}{2} + \sin \frac{A}{2} \sin \frac{C}{2} \\ = 2 (\cos \frac{A}{2} \cos \frac{C}{2} - \sin \frac{A}{2} \sin \frac{C}{2}) \\ o r 3 \sin (\frac{A}{2}) \sin (\frac{C}{2}) = \cos (\frac{A}{2}) \cos (\frac{C}{2}) \\ o r 3 \tan (\frac{A}{2}) \tan (\frac{C}{2}) = 1 \end{array}$
$o r 3 \sqrt{(\frac{(s - b) (s - c)}{s (s - a)})} \sqrt{\{\frac{(s - a) (s - b)}{s (s - c)}\}} = 1$
$\begin{array}{ll} o r 3 (s - b) = s \Rightarrow 2 s = 3 b \\ o r a + b + c = 3 b \Rightarrow a + c = 2 b \end{array}$
$\Rightarrow$ a,b,c are in A.P.