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Prove that `cos (x/2)=sqrt ((1/2)(1 + cosx))`

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elahusb | Student, College Freshman | eNoter

Posted June 9, 2012 at 1:38 PM via web

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Prove that `cos (x/2)=sqrt ((1/2)(1 + cosx))`

Tagged with identity, math, trigonometry

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justaguide | College Teacher | (Level 2) Distinguished Educator

Posted June 9, 2012 at 1:52 PM (Answer #1)

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The identity `cos (x/2)=sqrt ((1/2)(1 + cosx))` has to be proved.

`cos (x/2)=sqrt ((1/2)(1 + cosx))`

let `x/2= y` , this gives the identity to be proved as `cos y = sqrt((1 + cos 2y)/2)`

`sqrt((1 + cos 2y)/2)`

=> `sqrt((1 + 2*cos^2y -1 )/2)`

=> `sqrt((2*cos^2y)/2)`

=> `sqrt(cos^2y)`

=> `cos y`

This proves `cos y = sqrt((1 + cos 2y)/2)` .

If `x/2` is substituted for y it proves: `cos (x/2)=sqrt ((1/2)(1 + cosx))`

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oldnick | Valedictorian

Posted April 20, 2013 at 10:25 PM (Answer #3)

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`cos(x/2)=sqrt((cosx+1)/2)`

squaring boht sides:

`cos^2(x/2)=(cosx+1)/2=(cos^2(x/2)-sen^2(x/2)+1)/2=`

`cos^2(x/2)=(cos^2(x/2)+cos^2(x/2))/2=` `(2cos^2(x/2))/2=cos^2(x/2)`

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dylee | High School Teacher | eNoter

Posted June 9, 2012 at 6:02 PM (Answer #2)

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The general equatio is shown as this:
cos(a+b) = cosa*cosb - sina*sinb

when a = b = x/2, 
cosx = cos(x/2)*cos(x/2) - sin(x/2)*sin(x/2)
        = (cos(x/2))^2 - (sin(x/2))^2
        = (cos(x/2))^2 - (1-cos(x/2)^2)        ((siny)^2 + (cosy)^2 = 1)
        = 2cos(x/2)^2 - 1

cosx = 2(cos(x/2))^2 - 1
=> 2(cos(x/2))^2 = cosx + 1
=> (cos(x/2))^2 = (cosx+1)/2

Therefore, cos(x/2) = ((cosx + 1)/2)^(1/2)

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