Area of Circle

The circle is a plane shape that the distance of every point on its side with the center of the circle is always the same.. The radius of the circle is the distance of the center to the edge of the circle. The radius is denoted by r. The circle has a center line. The center line twice the length of the radius. The center line is denoted by diameter (d). d = 2 × r. The Circumference is the distance around the edge of the circle. Pi (the symbol is the Greek letter π) is: The ratio of the Circumference  to the Diameter of a Circle.

• π = circumference/diameter = 3,1415926535897323846....
• A quick and easy approximation to π is 22/7 = 3,1428571...

A circle with a center point C has the following sections.

• C is the center of the circle.
• AC = CB = r = radius of the circle.
• AB = diameter of the circle
• AB = AC + CB = 2 × r = 2R = 2 × radius
• If the radius of a circle = r and diameter d = 2 × r or r =  ½ × d

Area of ​​a circle is  π  times the square of the radius of the circle Area =

Circle Area = π r2

When the area of ​​a circle is expressed by the diameter :
Area = πr2  → (note: r = ½  x d)
          = π (½ d) 2
         = π ( ¼ d)2 
          = ¼ π d2 

Circle Area = ¼ π d2

Diameter = 2 x radius

Circumference =2πr or πd

Example :
1.  A circle has 21 cm radius. What is area and circumference ?
     Area = π r 2 
               = 22/7 x 212
               = 1.386 cm
    Circumference = 2πr
                               = 2 x 22/7 x 21
                               = 44 x 21
                               = 132 cm

2.  A circle has 616 cm area and π = 22/7. What is r  ?

     Area    =  π  x r2

     616     = 22/7 x r2

    22 x r2 = 616 x 7

    22r2     = 4312
        r2      = 4312/22
        r2     = 196
        r       =  √196
        r       = 14 cm

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How to Find the Prime Factors of a Number

A Prime Number can be divided evenly only by 1, or itself.  Example: 5 can only be divided evenly by 1 or 5, so it is a prime number. But 6 can be divided evenly by 1, 2, 3 and 6 so it is not a prime number (it is a composite number). A Composite Number can be divided evenly by numbers other than 1 or itself. Example: 9 can be divided evenly by 1, 3 and 9, so 9 is a composite number. But 7 can only be divided evenly by 1 and 7, so 7 is not a composite number (it is a Prime Number).  

If the integers p, p and p ¹ 0 dan p  ¹  1 only have 1 divider 1 and p, then p is called prime numbers. Integers other than primes are called composite numbers Examples of prime numbers: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, ...etc. . Examples of composite numbers: 4, 6, 8, 9, 10, 12, 14, 15, 16, 18, 20, 21, 22, 24, 25, 26, 27, 28, 30, 32, 33, 34, 35, 36, ...etc.

A factor that is a prime number. One of the prime numbers that, when multiplied, give the original number.
Example:

The prime factors of 35 are 5 and 7 (5×7=35, and 5 and 7 are prime numbers).

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Time Measurement Unit

As we all know, that the unit of time that there are various ranging from seconds, minutes, hours, days, weeks, months, years, and others. 1 hour = 60 minutes, 1 minute = 60 seconds to, 1 hour = 60 x 60 seconds = 3.600 seconds. 1 year = 12 months, 1 month = 30/31 days.

For example:
23 hours = ... minutes ?
23 hours = 23 x 60 minutes = 138 minutes.
43 minutes = ... second ?
43 minutes = 43 x 60 seconds = 258 seconds.
7 hours = ... second ?

7 hours = 7 x 60 x 60 seconds = 25 200 seconds. (View from hours to seconds the stairs down two times, so it must be multiplied by as much as 2 times the value 60 as well).

1 Week = 7 Days
1 Month = 28 to 31 Days
1 Month = 4 weeks
1 Semester = 6 Months
1 year = 365 to 366 Days
1 year = 52 weeks
1 year = 12 months
1 decade = 10 years
1 century = 100 years
1 millennium = 1,000 years

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Temperature Conversion

Temperature is a quantity that express the degree of heat and cold an object is a tool used to measure the temperature of the thermometer. In everyday life people tend to measure the temperature using the sense of touch. But with the development of the technology was created to measure the temperature with a thermometer valid.

The following table shows the temperature conversion formulas for conversions to and from the Celsius scale.

Example :
Temperature in the living room is 86⁰F . Calculate the temperature of living room in the C and R !

• C: R: F = 5: 4: 9,  F = 86 ⁰
• C/F= C = 5/9(86 ⁰  -32 ⁰ ) = 5/9 x 54 ⁰  = 212 ⁰ C
• R/F=4/9 =R = 4/9(86 ⁰  -32 ⁰ ) = 4/9 x 54 ⁰  = 24 ⁰ R
• So, 86 ⁰ F =30 ⁰ C =24 ⁰ R

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