Let's say I have this functional equation
For all $ x,y \in R$
And I have given with $f(2)=5$
If I proceed to find $f(1)$
By substituting $x=1$and $y=2$
Then I will be getting $f(1)=2$ by using the given relation [$f(2)=5$]
But if I substitute $x=1$and $y=1$
And on solving for $f(1)$.
I am getting two values for $f(1)$ i.e $2$ and $1$
Now what is wrong in second approach.