a) (5x^4 - 10x^3)/(10x^5) = 5x^3*(x - 2)/(5x^3*2x^2) = (x - 2)/(2x^2)
b) (a^2 + 4a + 4)/(4 - a^2) = (a + 2)^2 / [(2 - a)(2 + a)] = (a + 2)/(2 - a)