Solution - Quadratic equations

Step by step solution :

Step  1  :

Trying to factor by splitting the middle term

 1.1     Factoring  x²-40x+400 

The first term is,  x²  its coefficient is  1 .
The middle term is,  -40x  its coefficient is  -40 .
The last term, "the constant", is  +400 

Step-1 : Multiply the coefficient of the first term by the constant   1 • 400 = 400 

Step-2 : Find two factors of  400  whose sum equals the coefficient of the middle term, which is   -40 .

Step-3 : Rewrite the polynomial splitting the middle term using the two factors found in step 2 above,  -20  and  -20 
                     x² - 20x - 20x - 400

Step-4 : Add up the first 2 terms, pulling out like factors :
                    x • (x-20)
              Add up the last 2 terms, pulling out common factors :
                    20 • (x-20)
Step-5 : Add up the four terms of step 4 :
                    (x-20)  •  (x-20)
             Which is the desired factorization

Multiplying Exponential Expressions :

 1.2    Multiply  (x-20)  by  (x-20) 

The rule says : To multiply exponential expressions which have the same base, add up their exponents.

In our case, the common base is  (x-20)  and the exponents are :
          1 , as  (x-20)  is the same number as  (x-20)¹ 
 and   1 , as  (x-20)  is the same number as  (x-20)¹ 
The product is therefore,  (x-20)⁽¹⁺¹⁾ = (x-20)² 

Equation at the end of step  1  :

  (x - 20)2  = 0 

Step  2  :

Solving a Single Variable Equation :

 2.1      Solve  :    (x-20)² = 0 

  (x-20) ² represents, in effect, a product of 2 terms which is equal to zero

For the product to be zero, at least one of these terms must be zero. Since all these terms are equal to each other, it actually means :   x-20  = 0

Add  20  to both sides of the equation : 
                      x = 20

Supplement : Solving Quadratic Equation Directly

Solving    x2-40x+400  = 0   directly 

Earlier we factored this polynomial by splitting the middle term. let us now solve the equation by Completing The Square and by using the Quadratic Formula

Parabola, Finding the Vertex :

 3.1      Find the Vertex of   y = x²-40x+400

Parabolas have a highest or a lowest point called the Vertex .   Our parabola opens up and accordingly has a lowest point (AKA absolute minimum) .   We know this even before plotting  "y"  because the coefficient of the first term, 1 , is positive (greater than zero). 

 Each parabola has a vertical line of symmetry that passes through its vertex. Because of this symmetry, the line of symmetry would, for example, pass through the midpoint of the two  x -intercepts (roots or solutions) of the parabola. That is, if the parabola has indeed two real solutions. 

 Parabolas can model many real life situations, such as the height above ground, of an object thrown upward, after some period of time. The vertex of the parabola can provide us with information, such as the maximum height that object, thrown upwards, can reach. For this reason we want to be able to find the coordinates of the vertex. 

 For any parabola,Ax²+Bx+C,the  x -coordinate of the vertex is given by  -B/(2A) . In our case the  x  coordinate is  20.0000  

 Plugging into the parabola formula  20.0000  for  x  we can calculate the  y -coordinate : 
  y = 1.0 * 20.00 * 20.00 - 40.0 * 20.00 + 400.0
or   y = 0.000

Parabola, Graphing Vertex and X-Intercepts :

Root plot for :  y = x²-40x+400
Vertex at  {x,y} = {20.00, 0.00} 
x-Intercept (Root) :
One Root at  {x,y}={20.00, 0.00} 
Note that the root coincides with
the Vertex and the Axis of Symmetry
coinsides with the line  x = 0 

Solve Quadratic Equation by Completing The Square

 3.2     Solving   x²-40x+400 = 0 by Completing The Square .

 Subtract  400  from both side of the equation :
   x²-40x = -400

Now the clever bit: Take the coefficient of  x , which is  40 , divide by two, giving  20 , and finally square it giving  400 

Add  400  to both sides of the equation :
  On the right hand side we have :
   -400  +  400    or,  (-400/1)+(400/1) 
  The common denominator of the two fractions is  1   Adding  (-400/1)+(400/1)  gives  0/1 
  So adding to both sides we finally get :
   x²-40x+400 = 0

Adding  400  has completed the left hand side into a perfect square :
   x²-40x+400  =
   (x-20) • (x-20)  =
  (x-20)²
Things which are equal to the same thing are also equal to one another. Since
   x²-40x+400 = 0 and
   x²-40x+400 = (x-20)²
then, according to the law of transitivity,
   (x-20)² = 0

We'll refer to this Equation as  Eq. #3.2.1  

The Square Root Principle says that When two things are equal, their square roots are equal.

Note that the square root of
   (x-20)²   is
   (x-20)^2/2 =
  (x-20)¹ =
   x-20

Now, applying the Square Root Principle to  Eq. #3.2.1  we get:
   x-20 = √ 0

Add  20  to both sides to obtain:
   x = 20 + √ 0
The square root of zero is zero

This quadratic equation has one solution only. That's because adding zero is the same as subtracting zero.

The solution is:
   x  =  20 

Solve Quadratic Equation using the Quadratic Formula

 3.3     Solving    x²-40x+400 = 0 by the Quadratic Formula .

 According to the Quadratic Formula,  x  , the solution for   Ax²+Bx+C  = 0  , where  A, B  and  C  are numbers, often called coefficients, is given by :
                                     
            - B  ±  √ B²-4AC
  x =   ————————
                      2A

  In our case,  A   =     1
                      B   =   -40
                      C   =  400

Accordingly,  B²  -  4AC   =
                     1600 - 1600 =
                     0

Applying the quadratic formula :

               40 ± √ 0
   x  =    —————
                    2

The square root of zero is zero

This quadratic equation has one solution only. That's because adding zero is the same as subtracting zero.

The solution is:
  x = 40 / 2 = 20

One solution was found :