
Trigonometry Trigonometry Math Forum 
 LinkBack  Thread Tools  Display Modes 
October 8th, 2018, 03:16 AM  #1 
Newbie Joined: Oct 2018 From: Ohio Posts: 3 Thanks: 0  Verifying Trigonometric identity
Please help me figure out this problem. Cos(XY)/CosXCosY = 1 + TanXTanY I feel clueless. How do you verify this identity? Thanks! Last edited by skipjack; October 8th, 2018 at 04:20 AM. 
October 8th, 2018, 04:17 AM  #2 
Global Moderator Joined: Oct 2008 From: London, Ontario, Canada  The Forest City Posts: 7,946 Thanks: 1137 Math Focus: Elementary mathematics and beyond 
$\displaystyle \cos(xy)=\cos x\cos y+\sin x\sin y$

October 8th, 2018, 04:23 AM  #3 
Global Moderator Joined: Dec 2006 Posts: 20,751 Thanks: 2135 
cos(X  Y) = cos(X)cos(Y) + sin(X)sin(Y) is a standard identity.

October 8th, 2018, 04:29 AM  #4 
Newbie Joined: Oct 2018 From: Ohio Posts: 3 Thanks: 0 
That's only as far as I got. I know that certain standard identity. What I don't understand is how to verify the whole formula to = 1+TanXTanY How do you verify the whole equation? (Cos(X)Cos(Y)+Sin(X)Sin(Y))/ Cos(X)Cos(Y) = 1+Tan(X)Tan(Y) Is there something I cancel out??? I know 1+Tan(x)= Sec(X) Am I supposed to get the left side of the equation to Sec(X)??? I'm so confused on how to completely verify this. Last edited by skipjack; October 8th, 2018 at 02:38 PM. Reason: to add missing parentheses 
October 8th, 2018, 05:41 AM  #5 
Newbie Joined: Oct 2018 From: Ohio Posts: 3 Thanks: 0 
Wow. Never mind. I just had a eureka moment. I understand now. I was just waaaayyy overthinking this. Thanks everyone!!!! 
October 8th, 2018, 02:40 PM  #6 
Global Moderator Joined: Dec 2006 Posts: 20,751 Thanks: 2135 
The second identity you mentioned should be 1 + tan²(X) = sec²(X), but isn't needed.


Tags 
identity, trigonometric, verifying 
Thread Tools  
Display Modes  

Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Trigonometric identity  Lobinho  Trigonometry  7  March 31st, 2015 02:13 PM 
trigonometric identity  matemagia  Trigonometry  4  August 30th, 2012 06:40 PM 
Another trigonometric identity #2  OriaG  Trigonometry  2  August 13th, 2012 03:35 PM 
trigonometric identity  afrim  Trigonometry  8  October 1st, 2011 01:59 AM 
trigonometric identity  TG  Algebra  2  March 7th, 2008 03:22 PM 