Solution - Quadratic equations

Step by step solution :

Step  1  :

Trying to factor by splitting the middle term

 1.1     Factoring  x²-20x-1280 

The first term is,  x²  its coefficient is  1 .
The middle term is,  -20x  its coefficient is  -20 .
The last term, "the constant", is  -1280 

Step-1 : Multiply the coefficient of the first term by the constant   1 • -1280 = -1280 

Step-2 : Find two factors of  -1280  whose sum equals the coefficient of the middle term, which is   -20 .

For tidiness, printing of 12 lines which failed to find two such factors, was suppressed

Observation : No two such factors can be found !!
Conclusion : Trinomial can not be factored

Equation at the end of step  1  :

  x2 - 20x - 1280  = 0 

Step  2  :

Parabola, Finding the Vertex :

 2.1      Find the Vertex of   y = x²-20x-1280

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  10.0000  

 Plugging into the parabola formula  10.0000  for  x  we can calculate the  y -coordinate : 
  y = 1.0 * 10.00 * 10.00 - 20.0 * 10.00 - 1280.0
or   y = -1380.000

Parabola, Graphing Vertex and X-Intercepts :

Root plot for :  y = x²-20x-1280
Axis of Symmetry (dashed)  {x}={10.00} 
Vertex at  {x,y} = {10.00,-1380.00} 
 x -Intercepts (Roots) :
Root 1 at  {x,y} = {-27.15, 0.00} 
Root 2 at  {x,y} = {47.15, 0.00} 

Solve Quadratic Equation by Completing The Square

 2.2     Solving   x²-20x-1280 = 0 by Completing The Square .

 Add  1280  to both side of the equation :
   x²-20x = 1280

Now the clever bit: Take the coefficient of  x , which is  20 , divide by two, giving  10 , and finally square it giving  100 

Add  100  to both sides of the equation :
  On the right hand side we have :
   1280  +  100    or,  (1280/1)+(100/1) 
  The common denominator of the two fractions is  1   Adding  (1280/1)+(100/1)  gives  1380/1 
  So adding to both sides we finally get :
   x²-20x+100 = 1380

Adding  100  has completed the left hand side into a perfect square :
   x²-20x+100  =
   (x-10) • (x-10)  =
  (x-10)²
Things which are equal to the same thing are also equal to one another. Since
   x²-20x+100 = 1380 and
   x²-20x+100 = (x-10)²
then, according to the law of transitivity,
   (x-10)² = 1380

We'll refer to this Equation as  Eq. #2.2.1  

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

Note that the square root of
   (x-10)²   is
   (x-10)^2/2 =
  (x-10)¹ =
   x-10

Now, applying the Square Root Principle to  Eq. #2.2.1  we get:
   x-10 = √ 1380

Add  10  to both sides to obtain:
   x = 10 + √ 1380

Since a square root has two values, one positive and the other negative
   x² - 20x - 1280 = 0
   has two solutions:
  x = 10 + √ 1380
   or
  x = 10 - √ 1380

Solve Quadratic Equation using the Quadratic Formula

 2.3     Solving    x²-20x-1280 = 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   =   -20
                      C   =  -1280

Accordingly,  B²  -  4AC   =
                     400 - (-5120) =
                     5520

Applying the quadratic formula :

               20 ± √ 5520
   x  =    ——————
                      2

Can  √ 5520 be simplified ?

Yes!   The prime factorization of  5520   is
   2•2•2•2•3•5•23 
To be able to remove something from under the radical, there have to be  2  instances of it (because we are taking a square i.e. second root).

√ 5520   =  √ 2•2•2•2•3•5•23   =2•2•√ 345   =
                ±  4 • √ 345

  √ 345   , rounded to 4 decimal digits, is  18.5742
 So now we are looking at:
           x  =  ( 20 ± 4 •  18.574 ) / 2

Two real solutions:

 x =(20+√5520)/2=10+2√ 345 = 47.148

or:

 x =(20-√5520)/2=10-2√ 345 = -27.148

Two solutions were found :

1.  x =(20-√5520)/2=10-2√ 345 = -27.148
2.  x =(20+√5520)/2=10+2√ 345 = 47.148