SG. 1 The sum of two numbers is 50 , and their product is 25 . If the sum of their reciprocals is $a$, find $a$.
Let the two numbers be $x$ and $y$. $x+y=50$ and $x y=25$
$$
a=\frac{1}{x}+\frac{1}{y}=\frac{x+y}{x y}=2
$$