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   for example: 2 = 4 then 2 , 3 , 4 , 6 are proportion

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And
  2 is called the first proportion 3 is called the second proportion
  4 is called the third proportion 6 is called the fourth proportion

also 2 and 6 is called the extremes
      3 and 4 is called the means
               the product of extremes = the product of means

                                        2 X 6 = 3 X 4 = 12

Remark(2):

? If ( ?? = ?? )  , then (a x d = b x c).  (Cross multiplication)

          ?? ??

? If a x d = b x c , then a, b, c and d are proportional

Which of the following ratios represent the proportion:

1) 12 , 4                                  2) 10 , 6
      21 7                                      25 10

4 × 21 = 84                                10 × 10 = 100
12 × 7 = 84                                25 × 6 = 150

So, 12    =  4    (proportion)             So, 10 = 6   (not proportion)
             7
      21                                         25 10

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