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 Nov 21, 2018
 #1
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Not the first one 

(3x^2 - y)^2    =  (3x^2 - y) (3x^2 - y)  = 9x^4 - 3x^2y - 3x^2 + y^4 =

9x^4 - 6x^2 + y^4

 

Second one      (3x^2)^2 - y^2   =  9x^4 - y^2

 

Third one  9(x^2)^2 - y^2   = 9x^4 - y^2

 

Not the fourth one   (9x^2)^2 - y^2   = 81x^4 - y^2

 

Fifth one    (3x^2 + y) (3x^2 - y)   = 9x^4 +3x^2y - 3x^2y - y^2   = 9x^4 - y^2

 

So......First, Third, Fifth

 

cool cool cool

 Nov 21, 2018

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