Speed test. Solve as fast as you can

Level 2

Simplify 3 + 2 2 \sqrt{3 + 2\sqrt{2}} to surd form and you get a value of a + b a + \sqrt{b} where a a and b b are positive integer and b b is non-square. What is the value of b a \frac {b}{\sqrt{a}} ?


The answer is 2.

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2 solutions

For this to simplify, we know that 3 + 2 2 3+2\sqrt{2} can be rewritten in the form ( x + y ) 2 (x+\sqrt{y})^{2} .

So 3 + 2 2 = ( x + y ) 2 3+2\sqrt{2}=(x+\sqrt{y})^{2} for some rational numbers x , y x, y .

Expanding the RHS, we have: 3 + 2 2 = x 2 + 2 x y + y 3+2\sqrt{2}=x^{2}+2x\sqrt{y}+y .

Since 2 x y 2x\sqrt{y} is the only irrational term on the RHS and \(2\sqrt{2})\ is the only irrational term on the LHS, we know they have to be equal.

We have: \(2x\sqrt{y}=2\sqrt{2}\) which leads to x = 1 x=1 and y = 2 y=2 .

So 3 + 2 2 = ( 1 + 2 ) 2 3+2\sqrt{2}=(1+\sqrt{2})^{2} .

Let's plug this back in to the problem.

3 + 2 2 \sqrt{3+2\sqrt{2}}

= ( 1 + 2 ) 2 = \sqrt{(1+\sqrt{2})^{2}}

= 1 + 2 =1+\sqrt{2} .

So a = 1 , b = 2 a=1, b=2 . This means that our answer is b a = 2 1 = 2 1 = 2 \frac{b}{\sqrt{a}}=\frac{2}{\sqrt{1}}=\frac{2}{1}=\boxed{2} .

This took me a grand total of 5 seconds. (Mathcounts countdown round practice haha)

Budi Utomo
Feb 10, 2014

can simply to 1 + 2^(1/2) --> 2/1^(1/2) = 2/1 = 2

27 seconds

King Zhang Zizhong - 7 years, 1 month ago

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