Solution - Quadratic equations

Step by step solution :

Step  1  :

Equation at the end of step  1  :

  ((0 -  7r2) +  5r) +  3  = 0 

Step  2  :

Trying to factor by splitting the middle term

 2.1     Factoring  -7r²+5r+3 

The first term is,  -7r²  its coefficient is  -7 .
The middle term is,  +5r  its coefficient is  5 .
The last term, "the constant", is  +3 

Step-1 : Multiply the coefficient of the first term by the constant   -7 • 3 = -21 

Step-2 : Find two factors of  -21  whose sum equals the coefficient of the middle term, which is   5 .

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

Equation at the end of step  2  :

  -7r2 + 5r + 3  = 0 

Step  3  :

Parabola, Finding the Vertex :

 3.1      Find the Vertex of   y = -7r²+5r+3

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,Ar²+Br+C,the  r -coordinate of the vertex is given by  -B/(2A) . In our case the  r  coordinate is   0.3571  

 Plugging into the parabola formula   0.3571  for  r  we can calculate the  y -coordinate : 
  y = -7.0 * 0.36 * 0.36 + 5.0 * 0.36 + 3.0
or   y = 3.893

Parabola, Graphing Vertex and X-Intercepts :

Root plot for :  y = -7r²+5r+3
Axis of Symmetry (dashed)  {r}={ 0.36} 
Vertex at  {r,y} = { 0.36, 3.89} 
 r -Intercepts (Roots) :
Root 1 at  {r,y} = { 1.10, 0.00} 
Root 2 at  {r,y} = {-0.39, 0.00} 

Solve Quadratic Equation by Completing The Square

 3.2     Solving   -7r²+5r+3 = 0 by Completing The Square .

 Multiply both sides of the equation by  (-1)  to obtain positive coefficient for the first term:
 7r²-5r-3 = 0  Divide both sides of the equation by  7  to have 1 as the coefficient of the first term :
   r²-(5/7)r-(3/7) = 0

Add  3/7  to both side of the equation :
   r²-(5/7)r = 3/7

Now the clever bit: Take the coefficient of  r , 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 :
   3/7  +  25/196   The common denominator of the two fractions is  196   Adding  (84/196)+(25/196)  gives  109/196 
  So adding to both sides we finally get :
   r²-(5/7)r+(25/196) = 109/196

Adding  25/196  has completed the left hand side into a perfect square :
   r²-(5/7)r+(25/196)  =
   (r-(5/14)) • (r-(5/14))  =
  (r-(5/14))²
Things which are equal to the same thing are also equal to one another. Since
   r²-(5/7)r+(25/196) = 109/196 and
   r²-(5/7)r+(25/196) = (r-(5/14))²
then, according to the law of transitivity,
   (r-(5/14))² = 109/196

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
   (r-(5/14))²   is
   (r-(5/14))^2/2 =
  (r-(5/14))¹ =
   r-(5/14)

Now, applying the Square Root Principle to  Eq. #3.2.1  we get:
   r-(5/14) = √ 109/196

Add  5/14  to both sides to obtain:
   r = 5/14 + √ 109/196

Since a square root has two values, one positive and the other negative
   r² - (5/7)r - (3/7) = 0
   has two solutions:
  r = 5/14 + √ 109/196
   or
  r = 5/14 - √ 109/196

Note that  √ 109/196 can be written as
  √ 109  / √ 196   which is √ 109  / 14

Solve Quadratic Equation using the Quadratic Formula

 3.3     Solving    -7r²+5r+3 = 0 by the Quadratic Formula .

 According to the Quadratic Formula,  r  , the solution for   Ar²+Br+C  = 0  , where  A, B  and  C  are numbers, often called coefficients, is given by :
                                     
            - B  ±  √ B²-4AC
  r =   ————————
                      2A

  In our case,  A   =     -7
                      B   =    5
                      C   =   3

Accordingly,  B²  -  4AC   =
                     25 - (-84) =
                     109

Applying the quadratic formula :

               -5 ± √ 109
   r  =    ——————
                      -14

  √ 109   , rounded to 4 decimal digits, is  10.4403
 So now we are looking at:
           r  =  ( -5 ±  10.440 ) / -14

Two real solutions:

 r =(-5+√109)/-14=-0.389

or:

 r =(-5-√109)/-14= 1.103

Two solutions were found :

1.  r =(-5-√109)/-14= 1.103
2.  r =(-5+√109)/-14=-0.389