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Calculati:
[tex]( x^{-1} + y^{-1} )[ x^{-2} -( xy)^{-1} + y^{-2} ]=[/tex]


Răspuns :

= [ 1 /x + 1 /y ] · [ 1 /x²   - 1 / xy  + 1 /y² ] = formula binom 
de tipul   a³ +b³
= ( 1 /x) ³  + (1 /y) ³ = 
 = 1 /x³    + 1 / y³
[tex] (x^-^1+y^-^1)[x^-^2-(xy)^-^1+y^-^2]=( \frac{1}{x} + \frac{1}{y} )( \frac{1}{x^2} - \frac{1}{xy} + \frac{1}{y^2} )= \\ \\ = \frac{1}{x^3} - \frac{1}{x^2y} + \frac{1}{xy^2} + \frac{1}{x^2y} - \frac{1}{xy^2} + \frac{1}{y^3} = \frac{1}{x^3} + \frac{1}{y^3} [/tex]