Inspired by Ashish Siva

Algebra Level 4

If 3 + 3 + 6 + 2 \sqrt{3+\sqrt{3}+\sqrt{6}+\sqrt{2}} can be expressed in the form of a b + c d + e \sqrt{\dfrac{a}{b}}+\sqrt{\dfrac{c}{d}}+e than find out the value of a + b + c + d + e a+b+c+d+e , where a a and b b , and c c and d d are coprimes.


The answer is 9.

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3 solutions

Chew-Seong Cheong
Apr 16, 2016

Let 3 + 3 + 6 + 2 = ( x + y 2 + z 3 ) 2 = x + y 2 + z 3 \sqrt{3+\sqrt{3}+\sqrt{6}+\sqrt{2}} = \sqrt{(x+y\sqrt{2}+z\sqrt{3})^2} = x+y\sqrt{2}+z\sqrt{3} .

Squaring both sides, we have:

( x + y 2 + z 3 ) 2 = 3 + 3 + 6 + 2 x 2 + 2 y 2 + 3 z 2 + 2 x y 2 + 2 y z 6 + 2 z x 3 = 3 + 3 + 6 + 2 \begin{aligned} (x+y\sqrt{2}+z\sqrt{3})^2 & = 3+\sqrt{3}+\sqrt{6}+\sqrt{2} \\ x^2 + 2y^2 + 3z^2 + 2xy\sqrt{2} + 2yz\sqrt{6} + 2zx\sqrt{3} & = 3+\sqrt{3}+\sqrt{6}+\sqrt{2} \end{aligned}

Equating the coefficients, we have:

{ 2 x y = 1 . . . ( 1 ) 2 y z = 1 . . . ( 2 ) 2 z x = 1 . . . ( 3 ) \begin{cases} 2xy = 1 &...(1) \\ 2yz = 1 & ...(2) \\ 2zx = 1 &...(3) \end{cases}

From ( 1 ) ( 2 ) : x z = 1 x = z \dfrac{(1)}{(2)}: \quad \dfrac{x}{z} = 1 \quad \Rightarrow x = z .

Similarly, from ( 2 ) ( 3 ) : x = y \dfrac{(2)}{(3)}: \quad \Rightarrow x = y .

From ( 1 ) : 2 x y = 1 2 x 2 = 1 x = y = z = 1 2 (1): \quad 2xy = 1 \quad \Rightarrow 2x^2 = 1 \quad \Rightarrow x = y = z = \dfrac{1}{\sqrt{2}}

Therefore,

3 + 3 + 6 + 2 = x + y 2 + z 3 = 1 2 + 1 + 3 2 \begin{aligned} \sqrt{3+\sqrt{3}+\sqrt{6}+\sqrt{2}} & = x+y\sqrt{2}+z\sqrt{3} \\ & = \sqrt{\frac{1}{2}} + 1 + \sqrt{\frac{3}{2}} \end{aligned}

a + b + c + d + e = 1 + 2 + 3 + 2 + 1 = 9 \Rightarrow a + b + c + d + e = 1 + 2 + 3 + 2 + 1 = \boxed{9}

Atul Shivam
Apr 14, 2016

Sorry guys !!! No time for latex

Atul Shivam - 5 years, 2 months ago

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No problem . BUT good question.

A Former Brilliant Member - 5 years, 2 months ago

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Thank you abhay :-)

Atul Shivam - 5 years, 2 months ago
William Isoroku
Apr 18, 2016

Add the radical fractions then square it and the original equation. Then just equate the corresponding parts.

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