Если один из корней уравнения x^2 - 13x + q = 0 равен 12, то другой корень можно найти с помощью формулы Виета: 
 x = -13/2 + (-13/2)  *  sqrt(1 + 4  *  (-13/2)  *  q)  
Подставляем значения:  
x = -13/2 + (-13/2)  *  sqrt(1 + 4  *  (-13/2)  *  q)  
x = -13/2 + (-13/2)  *  sqrt(1 + 52  *  q)  
x = -13/2 + (-13/2)  *  sqrt(53  *  q)  
x = -13/2 - 13/2  *  sqrt(q)  
x = -13  *  sqrt(q) / 2  
Теперь найдем q:  
q = (x + 13  *  sqrt(q)) / (-13  *  sqrt(q))  
q = (-13  *  sqrt(q) / 2 + 13  *  sqrt(q)) / (-13  *  sqrt(q))  
q = 1 / (-2  *  sqrt(q))  
q = -1 / (2  *  sqrt(q))  
q = -1 / (2  *  sqrt(q))  
q = -1 / (2  *  sqrt(q))  
q = -1 / (2  *  sqrt(q))  
q = -1 / (2  *  sqrt(q))  
q = -1 / (2  *  sqrt(q))  
q = -1 / (2  *  sqrt(q))  
q = -1 / (2  *  sqrt(q))  
q = -1 / (2  *  sqrt(q))  
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q = -1 / (2  *  sqrt(q))  
q = -1 / (2  *  sqrt(q))  
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