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if arccos x+ arccos y + arccos z= ╥    then prove x^2 +y^2 +z^2+ 2*x*y*z  = 1

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aaina | Student | eNotes Newbie

Posted October 24, 2011 at 6:55 PM via web

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if arccos x+ arccos y + arccos z= ╥    then prove x^2 +y^2 +z^2+ 2*x*y*z  = 1

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sciencesolve | Teacher | (Level 3) Educator Emeritus

Posted November 28, 2011 at 1:51 AM (Answer #1)

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Put arccosx=m, arccosy=n and arccos y=p => `m+n+p=pi`

Subtract p both sides =>`m+n=pi-p`

calculate cosine value of both sides:


use the formula: `cos(m+n)=cos m*cos n-sin m*sin n`

`cos(pi-p)=cos pi*cos p+sin pi*sin p (cos pi=-1, sin pi=0)`

`cos m*cos n-sin m*sin n=-cos p`

cos m=cos(arccos x)=x

cos n=cos(arccos y)=y

cos p=cos(arccosz)=z

`sin m=sin(arccos x)=sqrt(1-x^2)`

`` `sin n=sqrt(1-y^2)`

`cos m*cos n-sin m*sin n=-cosp ` <=> `xy-sqrt((1-x^2)(1-y^2))=-z`


Raise to square both sides to remove the square root:


Remove the brackets:


Reduce the same quantity`x^2*y^2`  both sides


Add `x^2+y^2 ` both sides


Answer: `x^2+y^2+z^2+2xyz=1`  proves the identity `arccos x+arccos y+arccos z=pi` .

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