Home » Ask & Discuss » Mathematics
« Back to Discussion

Trignometry

New kid on the Block

Joined:	10 Feb 2012
Post:	3

10 Feb 2012 19:32:57 IST

0 People liked this

453

3cosx+2cos3x=cosy & 2sin+2sin3x=siny find cos2x

Engineering Entrance , JEE Main , JEE Main & Advanced , Mathematics , Trigonometry

3cosx+2cos3x=cosy & 2sin+2sin3x=siny find cos2x

Comments (2)

jagdish singh

Blazing goIITian

Joined: 19 Jan 2008

Posts: 1089

11 Feb 2012 08:26:58 IST

Like 0 people liked this

$\hspace{-16}\mathbf{3\cos x+2\cos 3x = \cos y.................(1)}$\\\\ $\mathbf{3\sin x+2\sin 3x = \sin y...................(2)}$\\\\ Now Squaring and Adding $\mathbf{(1)}$ and $\mathbf{(2)}$, We Get\\\\ $\mathbf{9+4+12.\cos (2x) = 1}$\\\\ $\boxed{\boxed{\mathbf{\cos (2x) = -1}}}$$

Manasi

Forum Expert

Joined: 1 Dec 2006

Posts: 2108

13 Feb 2012 11:18:15 IST

Like 0 people liked this

2sinx + 2sin3x=siny............r u sure , its 2 sin x and not 3 sinx???

if its 2 sin x, the question is getting un-necessarily lengthy, n then also, the answer is not guranteed.

for 3 sin x, the solution given by jagdish, is perfect :)

Quick Reply

Reply

	Some HTML allowed.
	Keep your comments above the belt or risk having them deleted.
	Signup for a avatar to have your pictures show up by your comment
	If Members see a thread that violates the Posting Rules, bring it to the attention of the Moderator Team