Question:
The sum of two numbers is 9. The sum of their reciprocals is 1/2. Find the numbers.
Solution:
Let one numbers be x then other (9 − x).
Then according to question
$\frac{1}{x}+\frac{1}{(9-x)}=\frac{1}{2}$
$\Rightarrow \frac{(9-x)+x}{x(9-x)}=\frac{1}{2}$
$\Rightarrow \frac{9}{x(9-x)}=\frac{1}{2}$
By cross multiplication
$\Rightarrow 18=x(9-x)$
$\Rightarrow x^{2}-9 x+18=0$
$\Rightarrow x^{2}-6 x-3 x+18=0$
$\Rightarrow(x-6) x-3(x-6)=0$
$\Rightarrow(x-6)(x-3)=0$
$\Rightarrow x=6,3$
Since, x being a number,
Therefore,
When x = 6 then
(9 − x) = (9 − 6) = 3
When x = 3 then
(9 − x) = (9 − 3) = 6
Thus, two consecutive number be either 3, 6 or 6, 3.