prove that: sin2pi/7 + sin4pi/7 + sin8pi/7 = (root7)/2

prove that: sin2pi/7 + sin4pi/7 + sin8pi/7 = (root7)/2


1 Answers

Ramesh V
70 Points
12 years ago

Lets take a = 2pi/7

7a =2pi
sin4a = sin(2pi-3a)
sin4a  = -sin3a
2sin2a.cos2a = 4sin3(a) -3sina
4sin a.cos a (1-2sin2(a)) = sin a(4sin2(a) - 3)
4cos a(1-2sin2(a)) = 4sin2(a) -3

On squaring both sides

16(1-sin2(a)) [1-2sin2(a))]^2 = (4sin2(a) -3)2
64sin6(a) - 112sin4(a) + 56sin2(a) -7 =0
it is cubic in sin2(a)
its roots are sin2(2pi/7) ,sin4(pi/7) ,sin2(8pi/7)
sum of roots =7/4
sin2pi/7*sin4pi/7 +sin4pi/7*sin8pi/7 +sin8pi/7*sin2pi/7 = 0
we can simply prove it by using 2sin a.sin b= cos(a-b) - cos(a+b)
& cos(2pi-theta) = cos theta

(sin2pi/7+sin4pi/7 +sin8pi/7)2=7/4
sin2pi/7+sin4pi/7 +sin8pi/7=[(7)1/2]/2


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Naga Ramesh
IIT Kgp - 2005 batch

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