Find all integers x for which there exists an integer y such that 1/x + 1/y = 1/7
There are 5 pairs that satisfy the equation as follows:
x = -42 and y = 6
x = 6 and y = -42
x =8 and y = 56
x = 14 and y = 14
x = 56 and y = 8
1/x + 1/y = 1/7
[ x + y ] / [xy] = 1/7
7x + 7y = xy
7y - xy = - 7x
y [ 7 - x ] = - 7x
y = [ 7x ] / [ x - 7 ]
Note that as x ⇒ ±inf, y ⇒ 7
y will be an integer when
x = -42 y = 6
{no need to test any x values less than this since y will be < 7}
x = 6 y = - 42
x = 8 y = 56
x = 14 y = 14
x = 56 y = 8
{no need to test any x values more than this since y will be > 7}