1.) y = 3x - 20 and y = -6x + 30*

A reasonable domain is d = 9 days

Step-by-step explanation:

Given:

The equation to show the growth of algae's radius is given as:

f(d) = 9(1.04)^d

Where, 'd' represents the number of days passed. So, it represents the domain of the given function f(d).

f(d) is the function representing the radius of algae after 'd' days.

Now, the radius of algae given after 'd' days = 12.81 mm

Therefore, plug in 12.81 for  f(d). This gives,

12.81=9(1.04)^dfrac{12.81}{9}=(1.04)^dtextrm{Taking log on both the sides, we get:}log (\frac{12.81}{9})=\log(1.04)^dtextrm{Using log property} \log a^b=b\log a \therefore \log (\frac{12.81}{9})=d\times\log(1.04)d=\frac{\log (\frac{12.81}{9})}{\log(1.04)}d=9\ days

Therefore, after 9 days, the radius of algae is 12.81 mm.

I hope I helped!