Use app×
Join Bloom Tuition
One on One Online Tuition
JEE MAIN 2025 Foundation Course
NEET 2025 Foundation Course
CLASS 12 FOUNDATION COURSE
CLASS 10 FOUNDATION COURSE
CLASS 9 FOUNDATION COURSE
CLASS 8 FOUNDATION COURSE
0 votes
11.8k views
in Trigonometric Functions by (46.2k points)

If A + B + C – 180°, then prove that:

(i) cos 2A + cos 2B – cos 2C = 1 – 4 sin A sin 5 cos C

(ii) sin A – sin B + sin C = 4 sin A/2 cos B/2 sin C/2

1 Answer

+1 vote
by (48.0k points)
selected by
 
Best answer

(i) L.H.S.
= cos 2A + cos 2B – cos 2C

= (cos 2A + cos 2B) – cos 2C

= 2 cos (A + B) cos (A – B) – cos 2C

= 2 cos (180° – C) cos (A – B) – (2 cos2 C – 1)

= -2 cos C cos (A – B) -2 cos2 C + 1

= 1 – 2 cos C cos (A – B) -2 cos2 C

= 1 – 2 cos C [cos (A – B) + cos C]

Welcome to Sarthaks eConnect: A unique platform where students can interact with teachers/experts/students to get solutions to their queries. Students (upto class 10+2) preparing for All Government Exams, CBSE Board Exam, ICSE Board Exam, State Board Exam, JEE (Mains+Advance) and NEET can ask questions from any subject and get quick answers by subject teachers/ experts/mentors/students.

...