+0  
 
+1
79
3
avatar+154 

Help plzzz

 Mar 18, 2020
 #1
avatar+23553 
+1

Trig identity

 

cos (x/2)   = +-  sqrt  ( (1+cos x)/2 )       Plug and chug   ....

 Mar 18, 2020
edited by ElectricPavlov  Mar 18, 2020
 #3
avatar+23553 
+1

You're told  'x' is in Q III     then   x/2 will be in Q II and be negative also

 

    using the identity       cos x/2  = +- sqrt   (   (1+(-2/3)) /2) = +- sqrt 1/6   =  +-  1/sqrt6  =  sqrt 6  / 6 

                                                                                                                     BUT it is   in Q II    - sqrt 6/6

ElectricPavlov  Mar 18, 2020
 #2
avatar+76 
0

I'll take a shot at this.

 

we now that cos x = -2/3

so cos x/2 = -2/3 x 1/2=-2/6=-1/3

 

Maybe -1/3?

 Mar 18, 2020

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