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81/100-x^2 = 0
-1*x^2 = -81/100 // : -1
x^2 = 81/100
x^2 = 81/100 // ^ 1/2
abs(x) = 9/10
x = 9/10 or x = -9/10
x in { 9/10, -9/10 }
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