Solution - Quadratic equations

Step by step solution :

Step  1  :

Trying to factor by splitting the middle term

 1.1     Factoring  y²-50y-600 

The first term is,  y²  its coefficient is  1 .
The middle term is,  -50y  its coefficient is  -50 .
The last term, "the constant", is  -600 

Step-1 : Multiply the coefficient of the first term by the constant   1 • -600 = -600 

Step-2 : Find two factors of  -600  whose sum equals the coefficient of the middle term, which is   -50 .

Step-3 : Rewrite the polynomial splitting the middle term using the two factors found in step 2 above,  -60  and  10 
                     y² - 60y + 10y - 600

Step-4 : Add up the first 2 terms, pulling out like factors :
                    y • (y-60)
              Add up the last 2 terms, pulling out common factors :
                    10 • (y-60)
Step-5 : Add up the four terms of step 4 :
                    (y+10)  •  (y-60)
             Which is the desired factorization

Equation at the end of step  1  :

  (y + 10) • (y - 60)  = 0 

Step  2  :

Theory - Roots of a product :

 2.1    A product of several terms equals zero. 

 When a product of two or more terms equals zero, then at least one of the terms must be zero. 

 We shall now solve each term = 0 separately 

 In other words, we are going to solve as many equations as there are terms in the product 

 Any solution of term = 0 solves product = 0 as well.

Solving a Single Variable Equation :

 2.2      Solve  :    y+10 = 0 

 Subtract  10  from both sides of the equation : 
                      y = -10

Solving a Single Variable Equation :

 2.3      Solve  :    y-60 = 0 

 Add  60  to both sides of the equation : 
                      y = 60

Supplement : Solving Quadratic Equation Directly

Solving    y2-50y-600  = 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   t = y²-50y-600

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  "t"  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,Ay²+By+C,the  y -coordinate of the vertex is given by  -B/(2A) . In our case the  y  coordinate is  25.0000  

 Plugging into the parabola formula  25.0000  for  y  we can calculate the  t -coordinate : 
  t = 1.0 * 25.00 * 25.00 - 50.0 * 25.00 - 600.0
or   t = -1225.000

Parabola, Graphing Vertex and X-Intercepts :

Root plot for :  t = y²-50y-600
Axis of Symmetry (dashed)  {y}={25.00} 
Vertex at  {y,t} = {25.00,-1225.00} 
 y -Intercepts (Roots) :
Root 1 at  {y,t} = {-10.00, 0.00} 
Root 2 at  {y,t} = {60.00, 0.00} 

Solve Quadratic Equation by Completing The Square

 3.2     Solving   y²-50y-600 = 0 by Completing The Square .

 Add  600  to both side of the equation :
   y²-50y = 600

Now the clever bit: Take the coefficient of  y , which is  50 , divide by two, giving  25 , and finally square it giving  625 

Add  625  to both sides of the equation :
  On the right hand side we have :
   600  +  625    or,  (600/1)+(625/1) 
  The common denominator of the two fractions is  1   Adding  (600/1)+(625/1)  gives  1225/1 
  So adding to both sides we finally get :
   y²-50y+625 = 1225

Adding  625  has completed the left hand side into a perfect square :
   y²-50y+625  =
   (y-25) • (y-25)  =
  (y-25)²
Things which are equal to the same thing are also equal to one another. Since
   y²-50y+625 = 1225 and
   y²-50y+625 = (y-25)²
then, according to the law of transitivity,
   (y-25)² = 1225

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
   (y-25)²   is
   (y-25)^2/2 =
  (y-25)¹ =
   y-25

Now, applying the Square Root Principle to  Eq. #3.2.1  we get:
   y-25 = √ 1225

Add  25  to both sides to obtain:
   y = 25 + √ 1225

Since a square root has two values, one positive and the other negative
   y² - 50y - 600 = 0
   has two solutions:
  y = 25 + √ 1225
   or
  y = 25 - √ 1225

Solve Quadratic Equation using the Quadratic Formula

 3.3     Solving    y²-50y-600 = 0 by the Quadratic Formula .

 According to the Quadratic Formula,  y  , the solution for   Ay²+By+C  = 0  , where  A, B  and  C  are numbers, often called coefficients, is given by :
                                     
            - B  ±  √ B²-4AC
  y =   ————————
                      2A

  In our case,  A   =     1
                      B   =   -50
                      C   =  -600

Accordingly,  B²  -  4AC   =
                     2500 - (-2400) =
                     4900

Applying the quadratic formula :

               50 ± √ 4900
   y  =    ——————
                      2

Can  √ 4900 be simplified ?

Yes!   The prime factorization of  4900   is
   2•2•5•5•7•7 
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).

√ 4900   =  √ 2•2•5•5•7•7   =2•5•7•√ 1   =
                ±  70 • √ 1   =
                ±  70

So now we are looking at:
           y  =  ( 50 ± 70) / 2

Two real solutions:

y =(50+√4900)/2=25+35= 60.000

or:

y =(50-√4900)/2=25-35= -10.000

Two solutions were found :

1.  y = 60
2.  y = -10