#### Jason

##### Well-known member

\(\displaystyle \dfrac{a}{b} + \dfrac{c}{d} \)

\(\displaystyle \dfrac{(a)(d) + (b)(c)}{(b)(d)} \) etc.. (Subtraction one has a minus instead of a plus.)

The speed at doing LCM depends on how good you are finding LCM - and that could be as fast as someone knowing the formula given in this post.