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Algebra Level pending

Consider a , b , c Z a,b,c \in \mathbb{Z}^* such that b b is square free and

( a + b c ) 2 = a + b c \left ( \dfrac{a+\sqrt{b}}{c} \right )^2 = a+\dfrac{\sqrt{b}}{c}

Evaluate a + b + c a + b + c .


The answer is 6.

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1 solution

Expanding the equation, we get a 2 + 2 a b + b c 2 = a + b c \dfrac{a^2+2a\sqrt{b}+b}{c^2} = a + \dfrac{\sqrt{b}}{c} . Separating the irrational part from the rational part, we get the system of two equations: { a 2 + b c 2 = a 2 a b c 2 = b c \begin{cases} \dfrac{a^2+b}{c^2} = a\\ \\ \dfrac{2a\sqrt{b}}{c^2} = \dfrac{\sqrt{b}}{c} \end{cases}

Analysing the irrational equation, we get 2 a c = 1 c = 2 a \dfrac{2a}{c} = 1 \leftrightarrow c = 2a . By plugging this into the rational equation, we yield a 2 + b 4 a 2 = a b = a 2 ( 4 a 1 ) \dfrac{a^2+b}{4a^2} = a \leftrightarrow b = a^2 (4a-1) .

But because b b is square free, we ought to have a = 1 a = 1 . Thus, we get b = 3 b = 3 and c = 2 c = 2 .

Our desired answer is 6. \boxed{6.}

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