Classic Algebra Question I

Algebra Level 4

a + c + 2 b + c = 1 + 2 + 3 \large\sqrt{a+\sqrt{c}+2\sqrt{b+\sqrt{c}}} = 1+\sqrt{2}+\sqrt{3}

The equation above holds true for integers a a , b b and c c . Find a + b + c a+b+c .


The answer is 35.

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2 solutions

Jian Hau Chooi
Jul 20, 2018

By taking square for both sides,

a + c + 2 b + c = 6 + 2 6 + 2 2 + 2 3 = 6 + 2 6 + 2 ( 2 + 3 ) + 2 ( 2 ) ( 3 ) = 6 + 24 + 2 5 + 24 \begin{aligned} a+\sqrt{c}+2\sqrt{b+\sqrt{c}} & = 6+2\sqrt{6}+2\sqrt{2}+2\sqrt{3} \\ & = 6+2\sqrt{6}+2\sqrt{(2+3)+2\sqrt{(2)(3)}} \\ & = 6+\sqrt{24}+2\sqrt{5+\sqrt{24}} \\ \end{aligned}

Clearly see that a = 6 , b = 5 , c = 24. a=6, b=5, c=24.

Thus, we will obtain a + b + c = 35 . a+b+c=\fbox{35}.

Chew-Seong Cheong
Jul 20, 2018

a + c + 2 b + c = 1 + 2 + 3 Squaring both sides a + c + 2 b + c = 1 + 2 + 3 + 2 ( 2 + 6 + 3 ) = 6 + 2 6 + 2 ( 2 + 3 ) 2 = 6 + 2 6 + 2 5 + 2 6 = 6 + 24 + 2 5 + 24 \begin{aligned} \sqrt{a+\sqrt c+2\sqrt{b+\sqrt c}} & = 1 + \sqrt 2 + \sqrt 3 & \small \color{#3D99F6} \text{Squaring both sides} \\ a+\sqrt c+2\sqrt{b+\sqrt c} & = 1 + 2 + 3 + 2\left(\sqrt 2 + \sqrt 6+ \sqrt 3\right) \\ & = 6 + 2\sqrt 6 + 2\sqrt{\left(\sqrt 2 + \sqrt 3\right)^2} \\ & = 6 + 2\sqrt 6 + 2\sqrt{5 + 2\sqrt 6} \\ & = 6 + \sqrt{24} + 2\sqrt{5 + \sqrt{24}} \end{aligned}

Therefore, a + b + c = 6 + 5 + 24 = 35 a+b+c = 6+5+24 = \boxed{35} .

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