An exponential decay function can be generically written as:
[tex]y=a\cdot b^x[/tex]The conditions for this function are:
1) The y-intercept is 4.
2) The values of y decrease by a factor of one half as x increases by 1.
The y-intercept corresponds to the value of y when x = 0, so we can express it as:
[tex]\begin{gathered} y=a\cdot b^x \\ 4=a\cdot b^0 \\ 4=a\cdot1 \\ a=4 \end{gathered}[/tex]This condition let us find the value of a.
The next condition will be used to find the value of b.
As x increases by 1, y decreases by one half.
We can write this as a quotient between consecutive values of y:
[tex]\begin{gathered} \frac{y(x+1)}{y(x)}=\frac{1}{2} \\ \frac{4\cdot b^{x+1}}{4\cdot b^x}=\frac{1}{2} \\ b^{x+1-x}=\frac{1}{2} \\ b^1=\frac{1}{2} \\ b=\frac{1}{2} \end{gathered}[/tex]Then, we can write the function as:
[tex]y=4\cdot(\frac{1}{2})^x[/tex]Answer: y = 4*(1/2)^x