Question 1162789
make X the subject of the formula
    A= X(2X-N)-X 
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a = 2x^2 - nx - x
2x^2 -(n+1)x - a = 0
It's a quadratic in x
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{{{x = (n+1)/4 + sqrt((n+1)^2 - 8a)/4}}}
{{{x = (n+1)/4 - sqrt((n+1)^2 - 8a)/4}}}