19 and 97 are primes right?

Find positive integers x , y , z x,y,z satisfying x < y < z x<y<z and 1 x 1 x y 1 x y z = 19 97 \dfrac{1}{x}-\dfrac{1}{xy}-\dfrac{1}{xyz}=\dfrac{19}{97} .

Submit your answer as the value of x + y + z x+y+z .


The answer is 151.

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1 solution

Ayush G Rai
Jun 18, 2016

The equation is equivalent to 19 x y z = 97 ( y z z 1 ) 19xyz=97(yz-z-1) or 19 x y z + 97 = 97 z × ( y 1 ) . 19xyz+97=97z\times (y-1).
19 19 and 97 97 are primes and so 97 97 should divide one of x , y , z . x,y,z. Also z z should divide 97 97 which means that z = 97 z=97 is prime.
From this it follows that y y divides 98 = 2 × 49 = 2 × 7 2 ; y 98=2\times 49=2\times 7^2;y being less than z = 97. z=97. The possibilities are y = 2 , y = 7 , y = 14 , y = 49. y=2,y=7,y=14,y=49.
If y = 2 , 19 × 2 × x = 2 × 97 98 = 96 y=2,19\times 2\times x=2\times 97-98=96 so that x x is not an integer.
If y = 7 , 19 x = 83 y=7,19x=83 so that x x is not an integer.
If y = 14 , 19 x = 90 y=14,19x=90 so that x x is not an integer.
If y = 49 , 19 × 49 x = 49 × 97 98. i . e . , x = 5. y=49,19\times 49x=49\times 97-98. i.e.,x=5.
Thus x = 5 , y = 49 , z = 97. x=5,y=49,z=97.
Therefore x + y + z = 5 + 49 + 97 = 151 . x+y+z=5+49+97=\boxed{151}.


Same solution!

Only took a lot of time to do it. :P

A Former Brilliant Member - 4 years, 12 months ago

took this from the amti book

abhishek alva - 4 years, 12 months ago

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Why do you want?You are just only to solve the problem and not question about the origin of this problem

Ayush G Rai - 4 years, 12 months ago

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