3 4 + 2 4 2
The above expression can be written in the form a + b c , where a , b and c are positive integers with c square-free. Find a + b + c .
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If you cannot see the perfect square, you can do it step by step.
Now, let 3 4 + 2 4 2 = p + q where p = a and q = b c
Square it:
3 4 + 2 4 2 = p 2 + q 2 + 2 p q
Since q contains a square root, we know that:
2 4 2 = 2 p q and 3 4 = p 2 + q 2
Square the first equation:
5 7 6 ( 2 ) = 4 p 2 q 2 p 2 q 2 = 2 8 8 q 2 = p 2 2 8 8
Substitute this into the second equation:
3 4 = p 2 + p 2 7 2 p 4 − 3 4 p 2 + 2 8 8 = 0 ( p 2 − 1 6 ) ( p 2 − 1 8 ) = 0 p 2 = 1 6 , 1 8 p = ± 4 , ± 1 8
Now, we know that p = a is a positive integer, therefore the only possible value of p is 4
Substitute p = 4 to find q :
2 4 2 = 2 ( 4 ) ( q ) q = 3 2
Therefore, 3 4 + 2 4 2 = 4 + 3 2
a = 4 , b = 3 , c = 2 and a + b + c = 4 + 3 + 2 = 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 and c = 2 . Further we have 2 a b = 2 4 or a b = 1 2 .
So we must have a = 4 and b = 3 or a = 3 and b = 4 .
Finally we just have to check which one of them satisfies a 2 + b 2 c = a 2 + 2 b 2 = 3 4
We see that a = 4 and b = 3 satisfies it.
Thus we have a + b + c = 4 + 3 + 2 = 9 .
( 3 4 + 2 4 2 ) = ( 4 + 3 2 ) 2 = ( 4 + 3 2 )
Comparing it with a + b c we get, a = 4 , b = 3 , c = 2 ,hence a + b + c = 9 .
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3 4 + 2 4 2 = 1 6 + 1 8 + 2 4 2 = ( 4 ) 2 + ( 3 2 ) 2 + ( 2 × 4 × 3 2 ) = ( 4 + 3 2 ) 2 = 4 + 3 2
∴ The answer is 4 + 3 + 2 = 9 .