In order to increase the circumference of a circle from 12π cm to 18π cm, by how much must the diameter increase?

The increase in the diameter is 6 cm.

Step-by-step explanation:

The circumference of a circle with diameter 'd' is given by :

Circumference=

Given: The circumference is increased from

It implies the increase in the circumference=, where is the previous diameter and

is the new diameter.

Hence, the increase in the diameter= 6 cm

Two solutions were found :

x =(8-√20)/2=4-√ 5 = 1.764

x =(8+√20)/2=4+√ 5 = 6.236

Step-by-step explanation:

Rearrange:

Rearrange the equation by subtracting what is to the right of the equal sign from both sides of the equation :

                    (x-4)^2-(5)=0

Step by step solution :

Step  1  :

1.1     Evaluate :  (x-4)2   =    x2-8x+16

Trying to factor by splitting the middle term

1.2     Factoring  x2-8x+11

The first term is,  x2  its coefficient is  1 .

The middle term is,  -8x  its coefficient is  -8 .

The last term, "the constant", is  +11

Step-1 : Multiply the coefficient of the first term by the constant   1 • 11 = 11

Step-2 : Find two factors of  11  whose sum equals the coefficient of the middle term, which is   -8 .

     -11    +    -1    =    -12

     -1    +    -11    =    -12

     1    +    11    =    12

     11    +    1    =    12

Observation : No two such factors can be found !!

Conclusion : Trinomial can not be factored

Equation at the end of step  1  :

 x2 - 8x + 11  = 0

Step  2  :

Parabola, Finding the Vertex :

2.1      Find the Vertex of   y = x2-8x+11

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   4.0000  

Plugging into the parabola formula   4.0000  for  x  we can calculate the  y -coordinate :

 y = 1.0 * 4.00 * 4.00 - 8.0 * 4.00 + 11.0

or   y = -5.000

Parabola, Graphing Vertex and X-Intercepts :

Root plot for :  y = x2-8x+11

Axis of Symmetry (dashed)  {x}={ 4.00}

Vertex at  {x,y} = { 4.00,-5.00}

x -Intercepts (Roots) :

Root 1 at  {x,y} = { 1.76, 0.00}

Root 2 at  {x,y} = { 6.24, 0.00}

Solve Quadratic Equation by Completing The Square

2.2     Solving   x2-8x+11 = 0 by Completing The Square .

Subtract  11  from both side of the equation :

  x2-8x = -11

Now the clever bit: Take the coefficient of  x , which is  8 , divide by two, giving  4 , and finally square it giving  16

Add  16  to both sides of the equation :

 On the right hand side we have :

  -11  +  16    or,  (-11/1)+(16/1)

 The common denominator of the two fractions is  1   Adding  (-11/1)+(16/1)  gives  5/1

 So adding to both sides we finally get :

  x2-8x+16 = 5

Adding  16  has completed the left hand side into a perfect square :

  x2-8x+16  =

  (x-4) • (x-4)  =

 (x-4)2

Things which are equal to the same thing are also equal to one another. Since

  x2-8x+16 = 5 and

  x2-8x+16 = (x-4)2

then, according to the law of transitivity,

  (x-4)2 = 5

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-4)2   is

  (x-4)2/2 =

 (x-4)1 =

  x-4

Now, applying the Square Root Principle to  Eq. #2.2.1  we get:

  x-4 = √ 5

Add  4  to both sides to obtain:

  x = 4 + √ 5

Since a square root has two values, one positive and the other negative

  x2 - 8x + 11 = 0

  has two solutions:

 x = 4 + √ 5

  or

 x = 4 - √ 5

Solve Quadratic Equation using the Quadratic Formula

2.3     Solving    x2-8x+11 = 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   =    -8

                     C   =   11

Accordingly,  B2  -  4AC   =

                    64 - 44 =

                    20

Applying the quadratic formula :

              8 ± √ 20

  x  =    —————

                   2

Can  √ 20 be simplified ?

Yes!   The prime factorization of  20   is

  2•2•5

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).

√ 20   =  √ 2•2•5   =

               ±  2 • √ 5

 √ 5   , rounded to 4 decimal digits, is   2.2361

So now we are looking at:

          x  =  ( 8 ± 2 •  2.236 ) / 2

Two real solutions:

x =(8+√20)/2=4+√ 5 = 6.236

or:

x =(8-√20)/2=4-√ 5 = 1.764

Two solutions were found :

x =(8-√20)/2=4-√ 5 = 1.764

x =(8+√20)/2=4+√ 5 = 6.236

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