Problem: If f is a function such that f(a+b)=f(a)1+f(b)1, find all possible values of f(2011).
Solution
Solution: Consider f(0)=f(0+0)=f(0)1+f(0)1 which gives [f(0)]2=2. Thus, f(0)=±2.
Let x=f(2011).
If f(0)=2 then x=f(2011)=f(2011+0)=f(2011)1+f(0)1=x1+21. So, x=2x2+x which yields 2x2−x−2=0.
Solving for x using the quadratic formula yields: x=221±1−4(2)(−2)=221±3 Hence, if f(0)=2 then either f(2011)=2 or f(2011)=−22.
However, suppose f(2011)=−22. Consider f(0)=f(2011+(−2011))=f(2011)1+f(−2011)1 which implies that 2=−2+f(−2011)1. Thus, f(−2011)=42.
But if we consider f(−2011)=f(−2011+0)=f(−2011)1+f(0)1, this means that 42=22+f(0)1. Thus, f(0)=−722 which is a contradiction. Thus for f(0)=2, f(2011)=2.
If f(0)=−2 then x=f(2011)=f(2011+0)=f(2011)1+f(0)1=x1−21. So, x=2x2−x which yields 2x2+x−2=0.
Solving for x using the quadratic formula yields: x=22−1±1−4(2)(−2)=22−1±3 Hence, if f(0)=−2 then either f(2011)=−2 or f(2011)=22.
However, suppose f(2011)=22. Consider f(0)=f(2011+(−2011))=f(2011)1+f(−2011)1 which implies that −2=2+f(−2011)1. Thus, f(−2011)=−42.
But if we consider f(−2011)=f(−2011+0)=f(−2011)1+f(0)1, this means that −42=−22+f(0)1. Thus, f(0)=722 which is a contradiction. Thus for f(0)=−2, f(2011)=−2.
So the possible values for f(2011) are ±2.
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