What is 1 over 7 plus 2 over 3 in simplest form? 1/7 + 2/3 (the common denominator is 21)    = (1 x 3)/(7 x 3) + (2 x 7)/(3 x 7)    = 3/21 + 14/21    = (3 + 14)/21    = 17/21, which is in its simp … lest form since both the numerator and the denominator do not have other common factors than 1.NJOY By writing an expression in its simplest form, it makes it as easy as possible to understand it. … For example, if you write out 12(x-y)y2x-2x2(x-3y-2x3)/(4)(x-y) Then you'll have very little luck understanding it. However, if you simplify this expression, you simply get 3.