Answer:
Eric's expression is :

and Andrea's is :

In Eric's expression, 20 represents the initial amount of substance with which he has started the experiment.
is the amount of substance left after each time period (in this case, each week).The variable w in this case represents the number of weeks.
Andrea's expression can be written as :

The one outside of parentheses represents the initial amount of the substance. The one inside of parentheses represents 100% of the original amount of the substance. 0.5 represents the 50% of the substance that is lost each time period. The variable w in this case represents the number of weeks.
Answer:
Equation in square form:

Extreme value:

Step-by-step explanation:
We are given

we can complete square

we can use formula


now, we can add and subtract 5^2



So, we get equation as

Extreme values:
we know that this parabola
and vertex of parabola always at extreme values
so, we can compare it with

where
vertex=(h,k)
now, we can compare and find h and k

we get
h=-5
k=-4
so, extreme value of this equation is

To solve an equation for one variable, we will utilize PEMDAS ( parentheses, exponents, multiplication/division, add/subtract) but backwards. So we will first add/subtract any numbers to the other side to get the variable by itself. We will then divide/multiply any numbers to get the variable by itself.