a) a/b+c/d=(a+c)/(bd) This is incorrect (unless b=1 and d=1) because a/bd does not equal a/b and c/bd does not equal c/d . b). a/b+c/d=(a+c)/(b+d) This is incorrect because you cannot add fractions merely by adding numerators and denominators.
c). a/b+c/d=(ad+bc)/(bd) This is correct: ad + bc/ bd = ad + bc/ bd
