testing header
Math Goodies is a free math help portal for students, teachers, and parents.
Free Math
Interactive Math Goodies Software

Buy Math Goodies Software
testing left nav
Math Forums @ Math Goodies
Math Forums @ Math Goodies
Home | Profile | Active Topics | Members | Search | FAQ
Save Password
Forgot your Password?

 All Forums
 Homework Help Forums
 Pre-Calculus and Calculus
 New Topic  Reply to Topic
 Printer Friendly
Author Previous Topic Topic Next Topic  

Senior Member

38 Posts

Posted - 05/16/2013 :  13:50:32  Show Profile  Reply with Quote
Given cos= -7/9 and tan < 0, find 2

The answer in the book shows (-562) / 81 but that is not what I got. My answer is (82)/ 9 Am I just completely wrong or is the solution in the book? Any help is appreciated.
Go to Top of Page

Senior Member

38 Posts

Posted - 05/16/2013 :  19:16:58  Show Profile  Reply with Quote
Okay, here is my work. I have worked it twice. I have two answers. I don't understand why i can't solve it like the first solution.

Solution 1
Using the pythagorean thm, I found the third side side to be 42.
So now, sin 2 = (42)/9 so sin = (82)/9

Solution 2
But if i use the sum formula for sin. I have sin( x + x) which equals sinxcosx + cosxsinx which equals (2)((42)/9)(-7/9) which equals what the books has for the solution (-562)/9

So i don't understand why I can't solve it like my top????
Go to Top of Page

Advanced Member

1469 Posts

Posted - 05/17/2013 :  08:48:10  Show Profile  Reply with Quote
In your Solution1, sin 2 is not equal to (42)/9
That is what sin is equal to.
So, since sin=(42)/9 and sin 2 = 2sincos, you get what you have done in the Solution2.

Edited by - the_hill1962 on 05/17/2013 08:52:18
Go to Top of Page
  Previous Topic Topic Next Topic  
 New Topic  Reply to Topic
 Printer Friendly
Jump To:
Math Forums @ Math Goodies © 2000-2004 Snitz Communications Go To Top Of Page
This page was generated in 0.05 seconds. Snitz Forums 2000
testing footer
About Us | Contact Us | Advertise with Us | Facebook | Blog | Recommend This Page

Copyright © 1998-2014 Mrs. Glosser's Math Goodies. All Rights Reserved.

A Hotchalk/Glam Partner Site - Last Modified 22 Dec 2014