Factor this

Number Theory Level pending

.Let A A and B B be integers such that A > 6 |A| > 6 and B > 6 |B| > 6

Find the number of unordered pairs ( A , B ) (A, B) such that A × B = 360 A \times B = 360 .

Definition: "Unordered" pairs means that ( 18 , 20 ) (18, 20) and ( 20 , 18 ) (20, 18) are the same. Count that as one pair, not two.


The answer is 12.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

2 solutions

Sabhrant Sachan
Jan 1, 2017

We don't really need to list all the pairs . Let's calculate the total number of pairs first .

A × B = 360 = 2 3 × 3 2 × 5. A \times B = 360 = 2^3 \times 3^2 \times 5 .

Total pairs = ( 3 + 1 ) × ( 2 + 1 ) × ( 1 + 1 ) = 24 (3+1)\times(2+1)\times(1+1) = 24 . These are all the ordered pairs with positive integer A A and B B . Unordered pairs will be 24 2 = 12 \dfrac{24}{2} = 12 . Out of these 12 12 pairs 6 6 of them will have A < 6 A < 6 . Total Pairs left = 6 = 6 . Since negative integers are also included , total pairs are 6 × 2 = 12 \boxed{6 \times 2 =12}

Denton Young
Dec 31, 2016

The pairs are:

( 8 , 45 ) , ( 8 , 45 ) , ( 9 , 40 ) , ( 9 , 40 ) , ( 10 , 36 ) , ( 10 , 36 ) , ( 12 , 30 ) , ( 12 , 30 ) , ( 15 , 24 ) , ( 15 , 24 ) , ( 18 , 20 ) , ( 18 , 20 ) (8, 45), (-8, -45), (9, 40), (-9, -40), (10, 36), (-10, -36), (12, 30), (-12, -30), (15, 24), (-15, -24), (18, 20), (-18, -20)

That's 12 pairs.

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...