# Trick to solve ??

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• September 30th 2008, 08:17 PM
F_S
Trick to solve ??
What is the trick to solve these ?

1. cos (2pi/7) + cos (4pi/7) + cos (6pi/7) = ?

2. sin (pi/10) * sin (13pi/10) = ?

I believe there has to be a simple trick to solve these, but am unable to figure that so far : ( .

Any clue ?
• October 2nd 2008, 03:30 AM
Greengoblin
Looks like you should plug it into a calculator from here. Most of the co-efficients of $\pi$ aren't obvious ones such as $\frac{\pi}{2}, \frac{\pi}{3}, \frac{3\pi}{2}$, and only one of them is top-heavy ( $\frac{13}{10}$), which can be simplified to $\frac{3}{10}$, which again is not a very obvious one. There's no algebraic tricks as far as I can see.