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Algebra Level 3

34 + 24 2 \large \sqrt{34 + 24\sqrt{2}}

The above expression can be written in the form a + b c a + b\sqrt{c} , where a , b a,b and c c are positive integers with c c square-free. Find a + b + c a+b+c .


This is one part of the set Fun with exponents


The answer is 9.

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4 solutions

Ashish Menon
Apr 12, 2016

34 + 24 2 = 16 + 18 + 24 2 = ( 4 ) 2 + ( 3 2 ) 2 + ( 2 × 4 × 3 2 ) = ( 4 + 3 2 ) 2 = 4 + 3 2 \sqrt{34 + 24\sqrt{2}}\\ = \sqrt {16 + 18 + 24\sqrt{2}}\\ = \sqrt {{(4)}^2 + {(3\sqrt{2})}^2 + (2 × 4 × 3\sqrt{2})}\\ = \sqrt {{(4 + 3\sqrt{2})}^2}\\ = 4 + 3\sqrt{2}


\therefore The answer is 4 + 3 + 2 = 9 4 + 3 + 2 = \boxed{9} .

Hung Woei Neoh
May 8, 2016

If you cannot see the perfect square, you can do it step by step.

Now, let 34 + 24 2 = p + q \sqrt{34+24\sqrt{2}} = p + q where p = a p = a and q = b c q = b\sqrt{c}

Square it:

34 + 24 2 = p 2 + q 2 + 2 p q 34+24\sqrt{2} = p^2 + q^2 + 2pq

Since q q contains a square root, we know that:

24 2 = 2 p q 24\sqrt{2} = 2pq and 34 = p 2 + q 2 34 = p^2 + q^2

Square the first equation:

576 ( 2 ) = 4 p 2 q 2 p 2 q 2 = 288 q 2 = 288 p 2 576(2) = 4p^2q^2\\ p^2q^2 = 288\\ q^2 = \dfrac{288}{p^2}

Substitute this into the second equation:

34 = p 2 + 72 p 2 p 4 34 p 2 + 288 = 0 ( p 2 16 ) ( p 2 18 ) = 0 p 2 = 16 , 18 p = ± 4 , ± 18 34 = p^2 + \dfrac{72}{p^2}\\ p^4 - 34p^2 + 288 = 0\\ (p^2 - 16)(p^2 - 18) = 0\\ p^2 = 16, 18\\ p = \pm 4, \pm \sqrt{18}

Now, we know that p = a p=a is a positive integer, therefore the only possible value of p p is 4 4

Substitute p = 4 p=4 to find q q :

24 2 = 2 ( 4 ) ( q ) q = 3 2 24\sqrt{2} = 2(4)(q)\\ q=3\sqrt{2}

Therefore, 34 + 24 2 = 4 + 3 2 \sqrt{34 + 24\sqrt{2}} = 4 + 3\sqrt{2}

a = 4 , b = 3 , c = 2 a=4,b=3,c=2 and a + b + c = 4 + 3 + 2 = 9 a+b+c = 4+3+2 = \boxed{9}

Here is a solution just in case you are not able to split the expression into a perfect square.

First notice that a < 6 a<6 and c = 2 c=2 . Further we have 2 a b = 24 2ab=24 or a b = 12 ab=12 .

So we must have a = 4 a=4 and b = 3 b=3 or a = 3 a=3 and b = 4 b=4 .

Finally we just have to check which one of them satisfies a 2 + b 2 c = a 2 + 2 b 2 = 34 a^2+b^2c=a^2+2b^2=34

We see that a = 4 a=4 and b = 3 b=3 satisfies it.

Thus we have a + b + c = 4 + 3 + 2 = 9 a+b+c=4+3+2=\boxed{9} .

Atul Shivam
Apr 12, 2016

( 34 + 24 2 ) = ( 4 + 3 2 ) 2 = ( 4 + 3 2 ) \sqrt{(34+24\sqrt{2})}=\sqrt{(4+3\sqrt{2})^2}=(4+3\sqrt{2})

Comparing it with a + b c a+b\sqrt{c} we get, a = 4 , b = 3 , c = 2 a=4,b=3,c=2 ,hence a + b + c = 9 a+b+c=\boxed{9} .

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