What binomial do you have to add to the polynomial x2+y squared–2xy+1 to get a polynomial: not containing the variable y? ?

What binomial do you have to add to the polynomial    to get a polynomial not containing the variable  ? we need to have a polynomial with opposite signs to all the y-terms to eliminate them.since we have term of  our polynomial needs to have so that  since we have term of  our polynomial needs to have so that  so we need to add:  

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