An amazing quadratic!

How many ordered pairs of integers ( x , y ) (x,y) are there such that

( x 10 ) ( x 12 ) = 2 y ? (x-10)(x-12) = 2^{y}?


The answer is 2.

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5 solutions

By the Fundamental Theorem of Arithmetic we must have both x 10 |x - 10| and x 12 |x - 12| as powers of 2 2 . Now the only two powers of 2 2 that differ by 2 2 are 2 2 and 4 4 . So since we always have that x 10 > x 12 x - 10 \gt x - 12 , we can have either

x 10 = 4 , x 12 = 2 x = 14 x - 10 = 4, x - 12 = 2 \Longrightarrow x = 14 or

x 10 = 2 , x 12 = 4 x = 8 x - 10 = -2, x - 12 = -4 \Longrightarrow x = 8 .

In both cases y = 3 y = 3 , so there are 2 \boxed{2} ordered pairs, namely ( 14 , 3 ) (14,3) and ( 8 , 3 ) (8,3) .

what is that theorem?

Vibha Gautam - 6 years, 6 months ago

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If 2 integers multiplied is a power of A then the integers must also be a power of A

Josh Banister - 6 years, 5 months ago

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When A is a prime, that is.

Jared Low - 6 years, 4 months ago
Atul Solanki
Nov 9, 2014

draw the graph of 2^y and x^2-22x+120 they for sure intersect only at two points therefore ordered pair of (x,y) are 2.

i donot understand

Vikash Kushvaha - 6 years, 6 months ago

But not necessarily they'll be lattice points.

tasmeem reza - 6 years, 4 months ago
Easton Bornemeier
Jan 24, 2015

A quadratic function intersects 2^y at 2 points knowing that the intersections stay in both the positive x and y directions

Roman Frago
Mar 13, 2015

This is just like a product of two consecutive even numbers that are both powers of two or the product of their negatives. So we have two sets, the positives and negatives of 2 and 4.

With both 2 and 4 positive, x = 14 , y = 3 x=14, y=3 and with both negative, x = 8 , y = 3 x=8, y=3 .

Mahmoud Amer
Jan 24, 2015

assume x-10=a and x-12=b when b = a+2 >>>1 a,b should be on the fourmela 2^n >>>>2 from 1 & 2 >>>>> (a,b) = (2,4) , ( -2,-4 ) let x-10 = 4 >>> x=14 >>> y+3 comp >>>

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