sums and differences of a rational algebraic expression 3/a + 2/ab + 1/abc another is 2x-1/6 + x+3/12 another is 2/a-b + 3/a+b another is 2a+b/a+b - 2a-b/a-b - 3a^2-b^2/a^2-b^2 another is 2x/2x^2+5x-3 + 5/x^2-9 another is x-3/12x^3-18x^2 - x+1/16x^2-24x