Solution - Quadratic equations

Step by step solution :

Step  1  :

Equation at the end of step  1  :

  ((0 -  7x2) +  5x) -  12  = 0 

Step  2  :

Step  3  :

Pulling out like terms :

 3.1     Pull out like factors :

   -7x² + 5x - 12  =   -1 • (7x² - 5x + 12) 

Trying to factor by splitting the middle term

 3.2     Factoring  7x² - 5x + 12 

The first term is,  7x²  its coefficient is  7 .
The middle term is,  -5x  its coefficient is  -5 .
The last term, "the constant", is  +12 

Step-1 : Multiply the coefficient of the first term by the constant   7 • 12 = 84 

Step-2 : Find two factors of  84  whose sum equals the coefficient of the middle term, which is   -5 .

For tidiness, printing of 18 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  3  :

  -7x2 + 5x - 12  = 0 

Step  4  :

Parabola, Finding the Vertex :

 4.1      Find the Vertex of   y = -7x²+5x-12

Parabolas have a highest or a lowest point called the Vertex .   Our parabola opens down and accordingly has a highest point (AKA absolute maximum) .    We know this even before plotting  "y"  because the coefficient of the first term, -7 , is negative (smaller 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   0.3571  

 Plugging into the parabola formula   0.3571  for  x  we can calculate the  y -coordinate : 
  y = -7.0 * 0.36 * 0.36 + 5.0 * 0.36 - 12.0
or   y = -11.107

Parabola, Graphing Vertex and X-Intercepts :

Root plot for :  y = -7x²+5x-12
Axis of Symmetry (dashed)  {x}={ 0.36} 
Vertex at  {x,y} = { 0.36,-11.11} 
Function has no real roots

Solve Quadratic Equation by Completing The Square

 4.2     Solving   -7x²+5x-12 = 0 by Completing The Square .

 Multiply both sides of the equation by  (-1)  to obtain positive coefficient for the first term:
 7x²-5x+12 = 0  Divide both sides of the equation by  7  to have 1 as the coefficient of the first term :
   x²-(5/7)x+(12/7) = 0

Subtract  12/7  from both side of the equation :
   x²-(5/7)x = -12/7

Now the clever bit: Take the coefficient of  x , which is  5/7 , divide by two, giving  5/14 , and finally square it giving  25/196 

Add  25/196  to both sides of the equation :
  On the right hand side we have :
   -12/7  +  25/196   The common denominator of the two fractions is  196   Adding  (-336/196)+(25/196)  gives  -311/196 
  So adding to both sides we finally get :
   x²-(5/7)x+(25/196) = -311/196

Adding  25/196  has completed the left hand side into a perfect square :
   x²-(5/7)x+(25/196)  =
   (x-(5/14)) • (x-(5/14))  =
  (x-(5/14))²
Things which are equal to the same thing are also equal to one another. Since
   x²-(5/7)x+(25/196) = -311/196 and
   x²-(5/7)x+(25/196) = (x-(5/14))²
then, according to the law of transitivity,
   (x-(5/14))² = -311/196

We'll refer to this Equation as  Eq. #4.2.1  

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

Note that the square root of
   (x-(5/14))²   is
   (x-(5/14))^2/2 =
  (x-(5/14))¹ =
   x-(5/14)

Now, applying the Square Root Principle to  Eq. #4.2.1  we get:
   x-(5/14) = √ -311/196

Add  5/14  to both sides to obtain:
   x = 5/14 + √ -311/196
In Math,  i  is called the imaginary unit. It satisfies   i²  =-1. Both   i   and   -i   are the square roots of   -1 


Since a square root has two values, one positive and the other negative
   x² - (5/7)x + (12/7) = 0
   has two solutions:
  x = 5/14 + √ 311/196 •  i 
   or
  x = 5/14 - √ 311/196 •  i 

Note that  √ 311/196 can be written as
  √ 311  / √ 196   which is √ 311  / 14

Solve Quadratic Equation using the Quadratic Formula

 4.3     Solving    -7x²+5x-12 = 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   =     -7
                      B   =    5
                      C   =  -12

Accordingly,  B²  -  4AC   =
                     25 - 336 =
                     -311

Applying the quadratic formula :

               -5 ± √ -311
   x  =    ——————
                      -14

In the set of real numbers, negative numbers do not have square roots. A new set of numbers, called complex, was invented so that negative numbers would have a square root. These numbers are written  (a+b*i) 

Both   i   and   -i   are the square roots of minus 1

Accordingly,√ -311  = 
                    √ 311 • (-1)  =
                    √ 311  • √ -1   =
                    ±  √ 311  • i

  √ 311   , rounded to 4 decimal digits, is  17.6352
 So now we are looking at:
           x  =  ( -5 ±  17.635 i ) / -14

Two imaginary solutions :

 x =(-5+√-311)/-14=(5-i√ 311 )/14= 0.3571+1.2597i
  or: 
 x =(-5-√-311)/-14=(5+i√ 311 )/14= 0.3571-1.2597i

Two solutions were found :

1.  x =(-5-√-311)/-14=(5+i√ 311 )/14= 0.3571-1.2597i
2.  x =(-5+√-311)/-14=(5-i√ 311 )/14= 0.3571+1.2597i