When the denominators are not divisible by one another, then to obtain a common denominator, it is requisite to multiply to-get her all of the denominators which will not divide any of the other denominators. For example: What is the sum of the tractions1/3,2/5,3/7, and 4/9?

In this case the first denominator will divide the last, but the others are prime to each other. Therefore, for the common denominator, multiply, together all but the first; or -

5x7x9 = 315 the common denominator; and -

315

÷

3

=

105,

common factor for the first fraction;

315

÷

5

=

63,

common factor for the second fraction;

315

÷

7

=

45,

common factor for the third; .

315

÷

9

=

35,

common factor for the fourth.

And, then -

1 x 105 = 105; 2 x 63 = 126 ; 3 x 45 = 135; 4 x 35 = 140; 3x 105 = 315' 5x63=315 7 x 45 = 315' 9 x 35 = 315

105 + 126 + 135 + 140 = 506 = 1 191

315 315 315 315 315 315