[Toán 8] Tính giá trị $x^2+y^2$

E

eye_smile

Ta có: ${\left( {x + y} \right)^2} = {x^2} + {y^2} + 2xy = {2^2} = 4$
${\left( {x - y} \right)^2} = {x^2} + {y^2} - 2xy = {\left( {\dfrac{{3\sqrt 2 }}{2}} \right)^2} = 4,5$
$ \to {\left( {x + y} \right)^2} + {\left( {x - y} \right)^2} = 2\left( {{x^2} + {y^2}} \right) = 8,5$
$ \to {x^2} + {y^2} = 4,25$
 
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