Answer is A
Just make sure you do know how to do them :)
Answer:

Step-by-step explanation:
Consider the given differential equation is



Taking all variables on right sides.


Let as assume,
and 
Find partial derivatives
and 
and 
Since
, therefore the given differential equation is exact.
The solution of the exact differential equation is






Answer: The answer is 314.28 cm² (approx.).
Step-by-step explanation: Given that an engineer is going to install a new water pipe. The diameter of this circular pipe is, d = 20 cm.
We need to find the area 'A' of the circular cross-section of the pipe.
Given, diameter of the circular section is

So, the radius of the circular cross-section will be

Therefore, cross-sectional area of the pipe is

Thus, the answer is 314.28 cm² (approx.).
By definition, we have

So, we have to solve two different equations, depending of the possible range for the variable. We have to remember about these ranges when we decide to accept or discard the solutions:
Suppose that 
In this case, the absolute value doesn't do anything: the equation is

We are supposing
, so we can accept this solution.
Now, suppose that
. Now the sign of the expression is flipped by the absolute value, and the equation becomes

Again, the solution is coherent with the assumption, so we can accept this value as well.
Sallys statement is always true
for example:
-6 + 2 = -4
but if I turn it around
2 - 6 = -4
same answer
-6 -6 = -12
swap around -6 - 6 = 12
same answer
again -3 + 6 = 3
and 6 - 3 = 3