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x in (-oo:+oo)
(x/2)*x+x = 210 // - 210
(x/2)*x+x-210 = 0
1/2*x^2+x-210 = 0
DELTA = 1^2-(-210*1/2*4)
DELTA = 421
DELTA > 0
x = (421^(1/2)-1)/(1/2*2) or x = (-421^(1/2)-1)/(1/2*2)
x = 421^(1/2)-1 or x = -(421^(1/2)+1)
x in { -(421^(1/2)+1), 421^(1/2)-1 }
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