How do you simplify $cos^2((2pi)/9) - sin^2((2pi)/9)$ ?

Question

2900 views around the world

You can reuse this answer
Creative Commons License

Answer 1 · 2016-04-12T09:31:18

$cos(80^o)=0.17365$ , nearly

$cos^2A-sin^2A=cos 2A$
Here, $A = 2pi/9 radian=2X180/9 = 40^o$ .
Answer is cos 2A = $cos(80^o)=0.17365$ , nearly

How do you simplify cos^2((2pi)/9) - sin^2((2pi)/9)cos^2((2pi)/9) - sin^2((2pi)/9)?