Please wait...
1800-212-7858 (Toll Free)
9:00am - 8:00pm IST all days

or

Thanks, You will receive a call shortly.
Customer Support

You are very important to us

For any content/service related issues please contact on this toll free number

022-62211530

Mon to Sat - 11 AM to 8 PM

if A+B+C=pi then prove the following parts: 1)cos2A+cos2B+cos2C= -1-4cosAcosBcosC 2)sin2A+sin2B+sin2c=4cosAsinBcosC 3)SinA+sinB-SinC=4sin(A/2)sin(B/2)cos(C/2)

Asked by 29th September 2013, 12:34 AM
Answered by Expert
Answer:
Please ask one question at a time. The solution to your first question is as follows.
 
cos2A + cos2B + cos2C = 2cos(A+B).cos(A-B) + 2cos^2C -1 
=2cos(180-C).cos(A-B) + 2cos^2C -1 
= - 2cosC.cos(A-B) +2cos^2C -1 
= -2cosC(cos(A-B) - cosC) -1 
= -2cosC(cos(A-B) - cos(180-(A+B)) ) -1 
= -2cosC(cos(A-B) + cos(A+B)) -1 
= -2cosC( 2cosA.cosB) -1 
= -4cosAcosBcosC -1
Answered by Expert 29th September 2013, 11:49 AM
Rate this answer
  • 1
  • 2
  • 3
  • 4
  • 5
  • 6
  • 7
  • 8
  • 9
  • 10

You have rated this answer /10

Your answer has been posted successfully!