Answer     
Find out the  what is the surface area of the pyramid .
To proof
Formula
Surface area of a square pyramid

Where a is the base edge and h be the height .
As given
A square pyramid has a base with side lengths each measuring 40 inches. The pyramid is 21 inches tall, with a slant height of 29 inches.
i.e Base edge = 40 inches
Height =  21 inches    
 Put values in the above formula 

solving



= 1600 + 80 × 29
= 3920 inches²
Therefore the surface area of the pyramid is 3,920 inches²
Hence proved
 
 
        
                    
             
        
        
        
23
This Website helps find percent 
https://percentagecalculator.net/
        
             
        
        
        
In this question we already know the value of 'b' so put the value b=2 in the equation
=|4(2)-8| + |-1-(2)^2| + 2(2)^3
=|8-8| + |-1-4| + 2(8)
=0 -5 +16
=11
 
        
             
        
        
        
Answer:
We can find if a critical point is a local minimum or maximum by looking at the second derivatives. 
Step-by-step explanation:
If you take the first derivative, you will find the slope at the given point, which if it is a minimum or a maximum will be 0. 
Then we take the second derivative. If that number is a positive number, then we have a local minimum. If it is a negative number, then it is a local maximum. 
 
        
             
        
        
        
Answer:
Option C step 3
Step-by-step explanation:
we have
-14x-2y=24 ------> equation A
14x+8y=-12 -----> equation B
step 1
Solve the system by elimination
Adds equation A and equation B
-14x-2y=24
14x+8y=-12
---------------------
-2y+8y=24-12
6y=12
The step 1 is correct
step 2
Solve for y
Divide by 6 both sides
6y/6=12/6
y=2
The step 2 is correct
step 3
Find the value of x
substitute the value of y in the equation A
-14x-2(2)=24
-14x-4=24
14x=-4-24
14x=-28
x=-2
The step 3 is not correct
therefore
Petro first make a mistake in Step 3