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Una equació de 2n  grau  a x squared plus b x plus c equals 0  és incompleta quan els coeficient b o c  són zero 


- Cas  box enclose bold b bold equals bold 0 end enclose 

          bold italic a bold italic x to the power of bold 2 bold plus bold italic c bold equals bold 0

                    bold italic a bold italic x to the power of bold 2 bold equals bold minus bold italic c bold rightwards arrow bold italic x to the power of bold 2 bold equals fraction numerator bold minus bold c over denominator bold a end fraction bold rightwards arrow bold italic x bold equals bold plus-or-minus square root of fraction numerator bold minus bold c over denominator bold a end fraction end root

   Exemples

   bold 2 bold italic x to the power of bold 2 bold minus bold 5 bold equals bold 0 bold rightwards arrow bold 2 bold italic x to the power of bold 2 bold equals bold 5 bold rightwards arrow bold italic x to the power of bold 2 bold equals bold 5 over bold 2 bold rightwards arrow bold italic x bold equals bold plus-or-minus square root of bold 5 over bold 2 end root

   x squared minus 9 equals 0 space space rightwards arrow space space space x squared equals 9 space space space rightwards arrow x equals plus-or-minus square root of 9 space space space rightwards arrow space box enclose space x equals plus-or-minus 3 end enclose space space

    4 x squared minus 25 equals 0 space space rightwards arrow space space space 4 x squared equals 25 space space rightwards arrow space space x squared equals 25 over 4 space rightwards arrow space space x equals plus-or-minus square root of 25 over 4 end root space rightwards arrow space box enclose x equals plus-or-minus 5 over 2 end enclose

     4 x squared plus 25 equals 0 space space space rightwards arrow space space space space space 4 x squared equals negative 25 space space rightwards arrow space space x squared equals negative 25 over 4 space space space rightwards arrow space space x equals plus-or-minus square root of negative 25 over 4 end root space space bottom enclose space N o space t é space s o l u c i ó end enclose


- Cas box enclose bold c bold equals bold 0 end enclose 

        bold italic a bold italic x to the power of bold 2 bold plus bold italic b bold italic x bold equals bold 0

                    bold italic x bold left parenthesis bold italic a bold italic x bold plus bold italic b bold right parenthesis bold equals bold 0 bold rightwards double arrow open curly brackets table attributes columnalign left end attributes row cell bold x subscript bold 1 bold equals bold 0 end cell row cell bold a bold x bold plus bold b bold equals bold 0 bold space bold space bold rightwards arrow bold space bold space bold space bold x subscript bold 2 bold equals fraction numerator bold minus bold b over denominator bold a end fraction end cell end table close  

    Exemples  

    bold 2 bold italic x to the power of bold 2 bold plus bold italic x bold equals bold 0 bold space bold rightwards arrow bold space bold 2 bold italic x to the power of bold 2 bold plus bold 1 bold italic x bold equals bold 0 bold rightwards arrow bold space bold italic x bold left parenthesis bold 2 bold italic x bold plus bold 1 bold right parenthesis bold space bold rightwards arrow open curly brackets table attributes columnalign left end attributes row cell bold x bold equals bold 0 bold space bold rightwards arrow bold space box enclose bold x subscript bold 1 bold equals bold 0 end enclose end cell row cell bold 2 bold x bold plus bold 1 bold equals bold 0 bold space bold rightwards arrow bold space box enclose bold x subscript bold 2 bold equals fraction numerator bold minus bold 1 over denominator bold 2 end fraction end enclose end cell end table close  

  3 x squared plus 5 x equals 0 space space space rightwards arrow space space x left parenthesis 3 x plus 5 right parenthesis equals 0 space space space rightwards arrow space open curly brackets table attributes columnalign left end attributes row cell box enclose bold x bold equals bold 0 end enclose end cell row cell 3 x plus 5 equals 0 space space rightwards arrow 3 x equals negative 5 space space rightwards arrow box enclose bold space bold x bold equals bold minus bold 5 over bold 3 end enclose end cell end table close

    x squared minus 5 x equals 0 space space space rightwards arrow space space x left parenthesis x minus 5 right parenthesis equals 0 space space space rightwards arrow space open curly brackets table attributes columnalign left end attributes row cell box enclose bold x bold equals bold 0 end enclose end cell row cell x minus 5 equals 0 space space rightwards arrow box enclose bold x bold equals bold 5 end enclose end cell end table close

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