
Calculus Calculus Math Forum 
 LinkBack  Thread Tools  Display Modes 
May 12th, 2008, 01:37 PM  #1 
Newbie Joined: May 2008 Posts: 2 Thanks: 0  finding values for trigonometric expression
Can someone help me with this question? Determine the exact value for each trigonometric expression, given sinA= 4/5, cos B= (8/17), [(pi)/2] < A < pi and (pi) < B < [3(pi)/2] Find 1) sin (AB) 2) Sin (A+B) 3) cos (A+B) 4) tan (AB) if you could just tell me one of the answers and how you got to it, or even just help me get it started, i can probably figure out the rest. I just dont even know where to begin! thank youuuuuu!! 
May 12th, 2008, 01:39 PM  #2 
Newbie Joined: May 2008 Posts: 2 Thanks: 0  correction
all the < signs should be "less than or equal to" ..sorry!

May 13th, 2008, 02:14 PM  #3 
Global Moderator Joined: Dec 2006 Posts: 20,921 Thanks: 2203 
You can use "<" instead of "â‰¤", as the values given for sin(A) and cos(B) are inconsistent with equality. sin(pi  A) = sin(A) = 4/5, so cos(A) = cos(pi  A) = 3/5 (by Pythagoras's theorem). cos(B  pi) = cos(B) = 8/17, so sin(B) = sin(B  pi) = 15/17 (by Pythagoras's theorem). 1) sin(A  B) â‰¡ sin(A)cos(B)  cos(A)sin(B) = (4/5)(8/17)  (3/5)(15/17) = 77/85. 2) sin(A + B) â‰¡ sin(A)cos(B) + cos(A)sin(B) = (4/5)(8/17) + (3/5)(15/17) = 13/85. (This is trigonometry, not calculus, so the topic will probably be moved from this section after a while.) 3) cos(A + B) â‰¡ cos(A)cos(B)  sin(A)sin(B) = (3/5)(8/17) + (4/5)(15/17) = 84/85. 4) tan(A  B) = sin(A  B)/cos(A  B) = (77/85)/(36/85) = 77/36. 

Tags 
expression, finding, trigonometric, values 
Thread Tools  
Display Modes  

Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
value of trigonometric expression [SOLVED]  chocolatesheep  Trigonometry  2  May 21st, 2013 07:20 PM 
Trigo (exact values and trigonometric equation)  choi2463  Trigonometry  3  May 7th, 2013 06:32 PM 
Calculate the trigonometric values  Lucida  Trigonometry  6  March 27th, 2012 11:45 AM 
Find some first values of the expression Sum_{j=1...n+1}...  ducnhuandoan  Number Theory  2  December 28th, 2011 11:16 PM 
Prove a trigonometric expression without a calculator  pisco  Trigonometry  1  June 10th, 2011 05:39 AM 