Let f be a real-valued function satisfying f(x+y)=f(x)+f(y) for all x, y. If f(1)=1/2, then the value of f(16) is: A. 16 B. 8 C. 4 D. 2
Question
Let $f$ be a real-valued function satisfying $f(x+y)=f(x)+f(y)$ for all $x$, y. If $f(1)=\frac{1}{2}$, then the value of $f(16)$ is:
A. 16
B. 8
C. 4
D. 2
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Answer
Given a real-valued function $f$ such that $f(x+y) = f(x) + f(y)$ for all real numbers $x$ and $y$, and given that $f(1) = \frac{1}{2}$, we are asked to find the value of $f(16)$.
Let's examine this step-by-step.
First, let's express $f(mx)$ where $m$ is an integer: $$ f(x + (m-1)x) = f(x) + f((m-1)x) $$ Using the given property repeatedly, $$ = f(x) + f(x + (m-2)x) = f(x) + f(x) + f((m-2)x) = \cdots = m f(x) + f(0) $$
Determining $f(0)$
Using the property $f(x+y) = f(x) + f(y)$: $$ \text{Putting } x = 0 \text{ and } y = 0, $$ $$ f(0+0) = f(0) + f(0) $$ $$ f(0) = 2f(0) \Rightarrow f(0) = 0 $$
Thus, the above expression simplifies to: $$ f(mx) = m f(x) \quad \text{where } m \text{ is an integer} $$
Finding $f(16)$
Given $f(1) = \frac{1}{2}$, $$ f(16 \times 1) = 16 f(1) $$ $$ f(16) = 16 \times \frac{1}{2} = 8 $$
Verification (Alternate )
Calculating successively: $$ f(2) = f(1) + f(1) = \frac{1}{2} + \frac{1}{2} = 1 $$ $$ f(4) = f(2) + f(2) = 1 + 1 = 2 $$ $$ f(8) = f(4) + f(4) = 2 + 2 = 4 $$ $$ f(16) = f(8) + f(8) = 4 + 4 = 8 $$
Therefore, the value of $f(16)$ is 8.
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