#### Jason

##### Well-known member

[math]\dfrac{a}{b} + \dfrac{c}{d} [/math]

[math]\dfrac{(a)(d) + (b)(c)}{(b)(d)} [/math] 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.