At the beginning of the semester, procrastinators reported an average of 0.8 symptoms, increasing at a rate of 0.45 symptoms, per week. Choose the function that models the average number of symptoms, $y$, after $x$ weeks.
$0.45=y+0.8$
$y=0.45 x+0.8$
$y=0.8 x+0.45$
$0.8=y+0.45 x$

Step 1 ：The question is asking for a function that models the average number of symptoms, y, after x weeks. Given that the average number of symptoms at the beginning of the semester is 0.8 and it increases at a rate of 0.45 symptoms per week, we can model this as a linear function.

Step 2 ：The initial number of symptoms (0.8) is the y-intercept and the rate of increase (0.45) is the slope of the function. Therefore, the function should be in the form of y = mx + b, where m is the slope and b is the y-intercept.

Step 3 ：In this case, m = 0.45 and b = 0.8. So, the function should be y = 0.45x + 0.8.

Step 4 ：Final Answer: The function that models the average number of symptoms, y, after x weeks is \(\boxed{y = 0.45x + 0.8}\).