Question 590909
{{{(9x^2-xy+y^2)+ (-x^2-8xy+9y^2)}}}


{{{9x^2-xy+y^2-x^2-8xy+9y^2}}} Remove unneeded parenthesis


{{{(9x^2-x^2)+(-xy-8xy)+(y^2+9y^2)}}} Group like terms


{{{8x^2-9xy+10y^2}}} Combine like terms


So {{{(9x^2-xy+y^2)+ (-x^2-8xy+9y^2)}}} fully simplifies to {{{8x^2-9xy+10y^2}}} 

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