(Headbang) Pff i've lost a 2h on internet trying to find how do you compute the LCM of 2 fractions

For example LCM of 2/3 and 1/4 is 2 , while LCM of 3/2 and 1/4 is 3/2

Why i need this?

Because i need to find the period of this

$\displaystyle x(t)=4cos(2\varpi t)+cos(3\varpi t) $