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 November 12th, 2012, 07:13 AM #1 Senior Member   Joined: Sep 2011 Posts: 395 Thanks: 0 Trigonometric Ratios The tables, or calculator, clearly shows that $\cos\pi$ or $\cos180^{\circ}=-1$. Is the books answer incorrect or did I miss something? Many thanks. Q. Using Mathematics Tables, write down the solutions of the equation $\cos\theta=1$ in the interval $0\leq\theta\leq2\pi$. Attempt: $\cos\theta=1=0^{\circ}&360^{\circ}=0&2\pi=$ Ans. $0&\pi&2\pi$
 November 12th, 2012, 10:52 AM #2 Senior Member     Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,211 Thanks: 520 Math Focus: Calculus/ODEs Re: Trigonometric Ratios Apparently, the book intended: $\cos^2(\theta)=1$ or $|\cos(\theta)|=1$ either that or the given solution is just wrong.
 November 12th, 2012, 10:56 AM #3 Senior Member   Joined: Sep 2011 Posts: 395 Thanks: 0 Re: Trigonometric Ratios Thanks for clearing that up. Much obliged.

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