If a , b , and c are positive integers satisfying the following equations:
5 a + 5 b + 2 a b 5 b + 5 c + 2 b c 5 c + 5 a + 2 c a = 9 2 = 1 3 6 = 2 4 4 ,
what is the value of b ?
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.
Alternatively, we can deduce from the same set of equations that 2 a + 5 = 1 9 , 2 b + 5 = 1 1 , 2 c + 5 = 2 7 and from there get b = 3
Subtracting pairs of equations, we get:
(c - a) * (2b + 5) = 44 or, c - a = 44 / (2b + 5)
(a - b) * (2c + 5) = 108 or, a - b = 108 / (2c + 5)
(b - c) * (2a + 5) = -152 or, b - c = -152 / (2a + 5)
Note that each of these 3 quantities is an integer as they are the differences of 2 integers, which must be integers. So, (2b + 5 ) | 44, (2c + 5) | 108, and (2a + 5) | 152
Since a > 0, b > 0, c > 0, we have 2a + 5 > 5, 2b + 5 > 5, 2c + 5 > 5
Now, 44 has factors 1, 2, 4, 11, 22 and 44. So, 2b + 5 can be equal to 11, 22 or 44. A little trial will yield the solution b = 3, a = 7, and c = 11
Problem Loading...
Note Loading...
Set Loading...
Multiply the first equation by 2 throughout we have:
1 0 a + 1 0 b + 4 a b = 1 8 4 ⇒ ( 2 a + 5 ) ( 2 b + 5 ) = 2 0 9 = ( 1 1 ) ( 1 9 )
Since a and b are integers, it means:
( 2 a ) ( 2 b ) = ( 1 1 − 5 ) ( 1 9 − 5 ) = ( 6 ) ( 1 4 ) ⇒ a b = ( 3 ) ( 7 )
Similarly,
1 0 b + 1 0 c + 4 b c = 2 7 2 ⇒ ( 2 b + 5 ) ( 2 c + 5 ) = 2 9 7 = ( 1 1 ) ( 2 7 )
⇒ b c = ( 3 ) ( 1 1 )
And
1 0 c + 1 0 a + 4 c a = 4 8 8 ⇒ ( 2 b + 5 ) ( 2 c + 5 ) = 5 1 3 = ( 1 9 ) ( 2 7 )
⇒ c a = ( 7 ) ( 1 1 )
As both b and 3 appear in Eqn 1 and Eqn 2, b = 3