CMR : a, x^2 + y^2 = (x+y)^2 – 2xy b, x^3+y^3=(x+y)^3-3xy(x+y) c, x^3-y^3=(x-y)^3+3xy(x-y)

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1. Giải thích các bước giải :

   `a)x^2+y^2`

   `=(x^2+2xy+y^2)-2xy`

   `=(x+y)^2-2xy`

   `b)x^3+y^3`

   `=(x^3+3x^2y+3xy^2+y^3)-(3x^2y+3xy^2)`

   `=(x+y)^3-3xy(x+y)`

   `c)x^3-y^3`

   `=(x^3-3x^2y+3xy^2-y^3)+(3x^2y-3xy^2)`

   `=(x-y)^3+3xy(x-y)`

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2. a, $x^2+y^2=(x+y)^2-2xy$

   $VP=x^2+2xy+y^2=x^2+y^2=VT$

   b, $x^3+y^3=(x+y)^3-3xy(x+y)$

   $VP=x^3+3x^2y+3xy^2+y^3-3x^2y-3xy^2=x^3+y^3=VT$

   c, $x^3-y^3=(x-y)^3+3xy(x-y)$

   $VP=x^3-3x^2y+3xy^2-y^3+3x^2y-3xy^2=x^3-y^3=VT$

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