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t^6=8-t^3

 May 14, 2016
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Solve for t:
t^6 = 8-t^3

 

Subtract 8-t^3 from both sides:
t^6+t^3-8 = 0

 

Substitute x = t^3:
x^2+x-8 = 0

 

Add 8 to both sides:
x^2+x = 8

 

Add 1/4 to both sides:
x^2+x+1/4 = 33/4

 

Write the left hand side as a square:
(x+1/2)^2 = 33/4

 

Take the square root of both sides:
x+1/2 = sqrt(33)/2 or x+1/2 = -sqrt(33)/2

 

Subtract 1/2 from both sides:
x = sqrt(33)/2-1/2 or x+1/2 = -sqrt(33)/2

 

Substitute back for x = t^3:
t^3 = sqrt(33)/2-1/2 or x+1/2 = -sqrt(33)/2

 

Taking cube roots gives (sqrt(33)/2-1/2)^(1/3) times the third roots of unity:
t = -(1/2 (1-sqrt(33)))^(1/3) or t = (1/2 (sqrt(33)-1))^(1/3) or t = (-1)^(2/3) (1/2 (sqrt(33)-1))^(1/3) or x+1/2 = -sqrt(33)/2

 

Subtract 1/2 from both sides:
t = -(1-sqrt(33))^(1/3)/2^(1/3) or t = (sqrt(33)-1)^(1/3)/2^(1/3) or t = ((-1)^(2/3) (sqrt(33)-1)^(1/3))/2^(1/3) or x = -1/2-sqrt(33)/2

 

Substitute back for x = t^3:
t = -(1-sqrt(33))^(1/3)/2^(1/3) or t = (sqrt(33)-1)^(1/3)/2^(1/3) or t = ((-1)^(2/3) (sqrt(33)-1)^(1/3))/2^(1/3) or t^3 = -1/2-sqrt(33)/2

 

Taking cube roots gives (-1/2-sqrt(33)/2)^(1/3) times the third roots of unity:
Answer: |  t = -(1-sqrt(33))^(1/3)/2^(1/3)    or    t = (sqrt(33)-1)^(1/3)/2^(1/3)    or    t = ((-1)^(2/3) (sqrt(33)-1)^(1/3))/2^(1/3)    or    t = (1/2 (-1-sqrt(33)))^(1/3)    or    t = -(1/2 (1+sqrt(33)))^(1/3)    or    t = -((-1)^(2/3) (1/2 (1+sqrt(33)))^(1/3))

 May 14, 2016

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