May 6th, 2014, 12:49 AM  #1 
Newbie Joined: May 2014 From: Sydney Posts: 16 Thanks: 0  trigonometry surd
What is the exact value of cos240? Shouldn't it be cos30(being closer to 270) Thus √3/2? However the answer says 1/2 Why is that? 
May 6th, 2014, 03:20 AM  #2 
Senior Member Joined: Apr 2014 From: Greater London, England, UK Posts: 320 Thanks: 155 Math Focus: Abstract algebra 
$$\cos240^\circ$$ $\displaystyle =\ \cos(180+60)^\circ$ $\displaystyle =\ \cos180^\circ\cos60^\circ  \sin180^\circ\sin60^\circ$ $\displaystyle =\ (1)\left(\frac12\right)\,\,(0)\left(\frac{\sqrt3}2\right)$ $\displaystyle =\ \frac12$ 

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